Well since the mathematical formula for difference of squares is most often written as
a^2-b^2 = (a-b)(a+b)
X ^2 + ?x - 25.
I believe the value that should replace the question mark would be 0, because by substituting 0, one will get the following:
X^2 +?x - 25
X^2 + 0x - 25
X^2 + 0 - 25
X^2 - 25
(X-5)(X+5).
Thus ? = 0. I believe is the solution.