8x - 7y = 10
In place of the x write ( ).
In place of the y write (-1):
         8(
).
In place of the y write (-1):
         8( ) - 7(-1) = 10
Now simplify:  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 (
) 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
,-1) is a 
solution to the equation 8x - 7y = 10.
Edwin