a) 5x - 7 + 2x = 21
5x + 2x - 7 = 21
7x - 7 = 21
7x = 21 + 7
7x = 28
x = 28:7
x = 4
b) 4x-3 = 2x+5
=> 4x - 2x = 5+3
2x = 8
x = 8:2
x = 4
\(a,5x-7+2x=21\)
\(\Rightarrow\left(5x+2x\right)-7=21\)
\(\Rightarrow7x-7=21\)
\(\Rightarrow7x=21+7\)
\(\Rightarrow7x=28\)
\(\Rightarrow x=28\div7\)
\(\Rightarrow x=4\)
\(b,4x-3=2x+5\)
\(\Rightarrow4x-2x=5+3\)
\(\Rightarrow2x=8\)
\(\Rightarrow x=8\div2\)
\(\Rightarrow x=4\)
\(5x-7+2x=21\)\(\Leftrightarrow7x=21+7\)\(\Leftrightarrow7x=28\)\(\Leftrightarrow x=4\)
\(4x-3=2x+5\)\(\Leftrightarrow4x-2x=5+3\)
\(\Leftrightarrow2x=8\)\(\Leftrightarrow x=4\)
