15x-3y=12

Y=5x-4

4x-y=-4

-8x+2y=2

15x-3y=12

y=5x-4

I'm also not sure what your question is, but this is what I got by solving it like a regular equation.

15x-3 (5x-4) = 12

15x-15x+12=12

0=12-12

0=0

4x-y=-4

-8x+2y=2

-y=-4-4x / : (-1)

y=4+4x

-8x+2y=2

-8x+2 (4+4x) = 2

-8x+8+8x=2

0=-6