Question 82890
let 3x+4y=-20 be the first equation
and x+y=-6 the second equation.

then, the second equation becomes x=-6-y.
plug it in the first equation.

3(-6-y)+4y=-20
-18-3y+4y=-20
-18+y=-20
y=-20+18
y=-2

and

x+(-2)=-6
x-2=-6
x=-6+2
x=-4


Then, check:

3(-4)+4(-2)=-20
-12+(-8)=-20
-12-8=-20
-20=-20