b: =>x(8-7)=-33

=>x=-33

c: =>-12x+60+21-7x=5

=>-19x=-76

hay x=4

d: =>-2x-2-x+5+2x=0

=>3-x=0

hay x=3

Bài 1:

a) \(=\dfrac{8}{15}\left(\dfrac{7}{13}+\dfrac{6}{13}\right)=\dfrac{8}{15}.1=\dfrac{8}{15}\)

b) \(=\dfrac{3.3-7-2.4}{12}=-\dfrac{6}{12}=-\dfrac{1}{2}\)

Bài 2:

 \(\dfrac{x}{2,7}=-\dfrac{2}{3,6}\Rightarrow x=\dfrac{\left(-2\right).2,7}{3,6}\Rightarrow x=-\dfrac{3}{2}\)

Bài 3:

\(\dfrac{x}{2}=\dfrac{y}{5}=\dfrac{x+y}{2+5}=-\dfrac{21}{7}=-3\)

\(\Rightarrow\left\{{}\begin{matrix}x=\left(-3\right).2=-6\\y=\left(-3\right).5=-10\end{matrix}\right.\)

d, x=-110

\(a,3\left(x-5\right)-4\left(x-3\right)=-12\)

\(\Leftrightarrow3x-15-4x+12=-12\)

\(\Leftrightarrow-x=-9\)

\(\Leftrightarrow x=9\)

Vậy x=9

\(c,20-\left(x-15\right)=x-15\)

\(\Leftrightarrow20-x+15=x-15\)

\(\Leftrightarrow2x=50\)

\(\Leftrightarrow x=25\)

Vậy x=25

12.(x-3)+7.(3-x)=15

12.x-12.3 +7.3-7.x=15

5.x - 15 =15

5x=30

x=6

Vậy x=6

mk nghĩ đề bài sai

(12x-36)+(21-7x)=15

12x-7x-36+21=15

5x-15=15

5x=15+15

5x=30

x=6

-12*(x-5)+7*(3-x)=15

60-12x+21-x=15

81-13x=15

13x=66

x=\(\frac{66}{13}\)