Is the equation x-y=10 and 2x-y=17?
Isolate the Y getting:
2x-17=y and x-10=y
Then bring the two equations together. Sub in the one equation for y.
2x-17 = x-10
The answer for x would be 7 (X=7)
Then pick an equation and sub in 7 for x to get your Y.
x-y=10
7-y=10
Subtract 7
-y=3
Divide the -1 from the y (Hopefully you learned why there's a 1 there)
y=-3
Answer is X=7 and Y=-3 or (7, -3)