8x - 7y = 10

In place of the x write ().
In place of the y write (-1):


         8() - 7(-1) = 10

Now simplify:  8() is 3 because the 8's cancel, and
-7(-1) becomes +7, so we have:

                3 + 7 = 10

                   10 = 10

Since the end result is the same number on both sides of
the equal sign, that tells us that (,-1) is a 
solution to the equation 8x - 7y = 10.

Edwin