Grabbed a system of equations to determine the solution.
y=2x+4.
and
y=x+9

Assuming your solving for x:
1. Rewrite y=x+9 for substitution: x=y-9
2. Plug in previous rewritten equation for x in the top one:
y=2x+4
y=2(y-9)+4
^Distribute
y=2y-18+4
y=2y-14
^minus y / plus 14
14=y
Hope this helps :))