A solution is when the two lines intersect, when they have the same (x,y) point. So we can say y=y which is the same as saying:
2x-15=5x
-3x-15=0
-3x=15
x=-5, now use either to solve for the y coordinate...y=5x becomes:
y=5(-5)=-25 so the solution is the point:
(-5, -25)