First Problem,

x=-4y

3x+2y=20

Take the first equation and put it into the second where the "x" is.

3(-4y)+2y=20

Solve for y. You get.

-12y+2y=20

-10y=20

y=-2

Second Problem

y= -x+6

3x+y=-10

Take the first equation and put it where the "y" is on the second equation.

After all that is what "y" equals.

3x+-x+6=-10

Solve for the Problem

2x+6=-10

2x=-16

x=-8

Double CheckingIt is actually (2, -7) and the second one is correct.