-7x+9y=18...............................eq1
4x+9y=-18...............................eq2
eq1-eq2 (to eliminate y)
-7x-4x+9y-9y=18+18
-11x=36
x=-36/11
substituting for x in eq1 we get
-7(-36/11)+9y=18
252/11+9y=18
9y=18 -252/11
9y=(198-252)/11
9y=-54/11
y=-54/(11)(9)
y=-6/11