im learning this too right now hope this helps
1.you get y by itself and put what y equals where y is in the other equation
y=2x. x+y=12
x+2x=12
3x=12. (divided 3 on both sides)
x=4
2.after you find x(or y) put it in one of the problems above and solve for whats missing which in this case is y
y=2(4) 4+y=12
y=8. -4. -4
y=8
