4x-6y-7x+8
combine like terms:
-3x-6y+8
If all you want to do is simplify, then that is the final answer.
However if we want to evaluate that when x=5 and y=2,
we substitute (5) in place of x and (2) in place of y
-3x-6y+8
-3(5)-6(2)+8
Then do all the operations:
-15-12+8
-19
