a) 2x + (-61) - (21 - 61) = 2x - 21 + (61 - 61) = 2x - 21.

b) (- 3 - x + 5) + 3 = (- 3 + 3) + 5 - x = 5 - x.

c) 11- (13 - x) + (13 - 11) = (11- 11) + (13- 13) + x = x

d) 25 - ( 15 - x + 303) + 303 = 25 - 15 + (303 - 303) + x = x + 10

a) \(x+5=20-\left(12-7\right)\)

\(\Rightarrow x+5=20-5\)

\(\Rightarrow x+5=15\)

\(\Rightarrow x=15-5\)

\(\Rightarrow x=10\)

b) \(15-\left(3+2x\right)=2^2\)

\(\Rightarrow3+2x=15-4\)

\(\Rightarrow3+2x=11\)

\(\Rightarrow2x=11-3\)

\(\Rightarrow2x=8\)

\(\Rightarrow x=\dfrac{8}{2}\)

\(\Rightarrow x=4\)

c) \(-11-\left(19-x\right)=50\)

\(\Rightarrow19-x=-11-50\)

\(\Rightarrow19-x=-61\)

\(\Rightarrow x=61+19\)

\(\Rightarrow x=80\)

d) \(159-\left(25-x\right)=43\)

\(\Rightarrow25-x=159-43\)

\(\Rightarrow25-x=116\)

\(\Rightarrow x=25-116\)

\(\Rightarrow x=-91\)

e) \(\left(79-x\right)-43=-\left(17-52\right)\)

\(\Rightarrow\left(79-x\right)-43=52-17\)

\(\Rightarrow79-x-43=35\)

\(\Rightarrow36-x=35\)

\(\Rightarrow x=1\)

f) \(\left(7+x\right)-\left(21-13\right)=32\)

\(\Rightarrow7+x-8=32\)

\(\Rightarrow x-1=32\)

\(\Rightarrow x=32+1\)

\(\Rightarrow x=33\)

g) \(-x+20=-15+8+13\)

\(\Rightarrow-x+20=6\)

\(\Rightarrow x=20-6\)

\(\Rightarrow x=14\)

h) \(-\left(-x+13-142\right)+18=55\)

\(\Rightarrow x-13+142+18=55\)

\(\Rightarrow x+147=55\)

\(\Rightarrow x=55-147\)

\(\Rightarrow x=-92\)

a) 22 + (2x -13) = 83 => 2x -13 = 61 => x = 37.

b) 51 - (-12 + 3x) = 27 => 63 - 3x = 27 => x = 12.

c) - (2x + 2) + 21 = - 23 => 2x + 2 = 44 => x = 21.

d) 25 - (25 - x) = 0 => 25 - 25 + x = 0 => x = 0.

1,

a,\(2020-\left(249+2020\right)+\left(249-573\right)\)

\(=2020-249-2020+249-573\)

\(=-573\)

b,\(\left|-257\right|+\left(-3\right)^0-\left(18+257\right)\)

\(=257+1-18-257\)

\(=1-18=-17\)

\(c,25.\left(85-47\right)-85.\left(47+25\right)\)

\(=25.85-25.47-47.85+85.25\)

\(=85.\left(25-47+25\right)-25.47\)

\(=85.3-25.47\)

\(=-920\)

2,

\(a,15-5.\left(x+2\right)=-30\)

\(=>5.\left(x+2\right)=15+30=45\)

\(=>x+2=\frac{45}{5}=9\)

\(=>x=7\)

\(b,\left(x+2\right)^2+5=105\)

\(=>\left(x+2\right)^2=100\)

\(=>\left(x+2\right)^2=10^2\)

\(=>x+2=10\)

\(=>x=8\)

\(c,\left|2x-5\right|-\left(-6\right)=11\)

\(=>\left|2x-5\right|=11-6=5\)

\(=>\orbr{\begin{cases}2x-5=5\\2x-5=-5\end{cases}}\)

\(=>\orbr{\begin{cases}2x=5-5=0\\2x=-5+5=0\end{cases}=>x=0}\)

`Answer:`

a. \(x+12=3\Leftrightarrow x=3-12\Leftrightarrow x=-9\)

b. \(2x-15=21\Leftrightarrow2x=21+15\Leftrightarrow2x=36\Leftrightarrow x=36:2\Leftrightarrow x=18\)

c. \(13-3x=4\Leftrightarrow-3x=4-13\Leftrightarrow-3x=-9\Leftrightarrow x=-9:-3\Leftrightarrow x=3\)

d. \(2\left(x-2\right)+4=12\Leftrightarrow2x-4+4=12\Leftrightarrow2x=12\Leftrightarrow x=12:2\Leftrightarrow x=6\)

e. \(15-3\left(x-2\right)=21\Leftrightarrow15-3x+6=21\Leftrightarrow-3x=21-15-6\Leftrightarrow-3x=0\Leftrightarrow x=0\)

g. \(25+4\left(3-x\right)=1\Leftrightarrow25+12-4x=1\Leftrightarrow37-4x=1\Leftrightarrow-4x=-36\Leftrightarrow x=9\)

h. \(3x+12=2x-4\Leftrightarrow3x-2x=-4-12\Leftrightarrow x=-16\)

i. \(14-3x=\left(-x\right)+4\Leftrightarrow-3x+x=4-14\Leftrightarrow-2x=10\Leftrightarrow x=5\)

k. \(2\left(x-2\right)+7=x-25\Leftrightarrow2x-4+7=x-25\Leftrightarrow2x-x=-25-3\Leftrightarrow x=-28\)