we have the system
8x-4y=-12 -----> equation A
8x+y=-17 -----> y=-17-8x -----> equation B
substitute equation B in equation A
8x-4(-17-8x)=-12
solve for x
8x+68+32x=-12
40x=-12-68
40x=-80
x=-2
Find the value of y
y=-17-8(-2)
y=-17+16
y=-1
the solution is
x=-2
y=-1
Example
8x-4y=-12 -----> equation A
8x+5y=-17 -----> equation B
isolate the variable y in equation B (or equation A)
5y=-17-8x
y=-(17/5)-(8/5)x
substitute in equation A
8x-4[-(17/5)-(8/5)x ]=-12
solve for x