Question

what is

2y+6=x
y-3x=-13

Answer 1

if you plug 2y + 6 in x on the second equation, it will be 
y-3(2y+6) = -13
y-6y-18 = -13
-5y = 5
y=-1

then you can plug in y in any of the equations to solve for x
(I'll do the first one)
2(-1)+6=x
-2+6=x
x=4

x=4 y=-1