5x+6y=15
y=-4x-7
To solve this, I would first substitute -4x-7 for y
5x+6(-4x-7)=15
Then distribute the terms in the 6(-4x-7) part
5x+-24x-42=15
Then simplify
-19x-42=15
Then add 42 to both sides
-19x=57
Divide -24 to both sides
x=57/-19 or -3
Then y=-4(-3)-7
y=12-7
y=5
