Explanation:
What is X+Y?
First note that if x + y = xy then neither x nor y can be equal to 1.
That being said, as a previous user already noted y = x/(x-1) and hence x - y = x - x(x-1) = x(x-2)/(x-1). Assume x - y = D. Then your question becomes what can D be so that the equation x(x-2)/(x-1) = D has a solution different than 1.
Rewrite this equation as x(x-2) = D(x-1) and note that x(x-2) = (x-1)^2 - 1. Replace x - 1 with z. So your question is what can D be so that the equation z^2 - 1 = Dz has a nonzero solution? Given that 0 cannot be a solution anyway of the previous equation. The question is really, for which D does the quadratic equation z^2 - Dz - 1 = 0 have solutions? By elementary 9th grade math, this equation has a solution when its discriminant D^2 + 4 is nonnegative. But D^2 + 4 is strictly positive regardless of the value of D. Hence D can be anything.
How to solve your question
Your question is
(+)−7+8
Simplify
1. Eliminate redundant parentheses
(+)−7+8
+−7+82
Add the numbers
+−7+8
++1
Solution
++1