Solution = (5,-5)
Step-by-step: for substitution
Isolate the x in 3x+6y=-15
===> x=-2y-5
Substitute the x in -7x-3y=-20 for -2y-5.
-7(-2y-5)-3y=-20
14y+35-3y=-20
11y=-55
Y=-5
Substitute the y in 3x+6y=-15 for -5
3x+6(-5)=-15
3x-30=-15
3x=15
X=5
(5,-5)