Swap sides on the second equation and add it to the first.
.. (-x) +(-3) = (9y +15) +(-9y -7x)
.. -x -3 = 15 -7x . . . . eliminate parentheses
.. 6x = 18 . . . . . . . . . add 7x +3
.. x = 3
Solve either equation for y.
.. -3 = 9y +15 . . . substitute for x in the first equation
.. -18 = 9y . . . . . . subtract 15
.. -2 = y . . . . . . . . divide by 9
The solution is (x, y) = (3, -2).