solve by elimination
2r - 9s = -17
9r + 2s = 51
18 r - 81 s = - 153
18 r + 4 s = 102
--------------------
0 r - 85 s = - 255
s = 255/85 = 3
now go back and get r
2r - 9s = -17
9r + 2s = 51
18 r + 4 s = 102
--------------------
0 r - 85 s = - 255
s = 255/85 = 3
now go back and get r