How do i solve the system of equations using substitution?

You plug in the y= equation into the y in the other equation and solve, once you get x you plug that in and solve for y

for example:
y=4x  and  30=2y+2x
so: 30=2(4x)+2x
30=8x+2x
30=10x
3=x
then, you plug 3 (x) in for x in either of the starting equations:
30=2y+2(3)
30=2y+6
24=2y
12=y
and get your   (3,12)

Main idea on left, details on right