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

Y= -2x-1 the negative in front of the 2 is because the slope is negative and the -1 is because it’s down 1 on the y axis.