Answer :

3x-4y=18                               
+3x   +3x
-----------------
-4y=3x+18
÷-4    ÷-4
----------------
y=-3/4x-6


x+3y=-7
-x    -x
------------
3y=-x-7
 ÷3    ÷3
--------------
y=-x-7/3

I think but not sure :)