(85 + 84 + 78 + x) / 4 = 86
(247 + x) / 4 = 86
247 + x = 86 * 4
247 + x = 344
x = 344 - 247
x = 97 <=== her score was 97
We have

so this is indeed a difference of squares. Factorizing would give

making the answer D.
We can further factor the first term here and write

but that's clearly out of the scope of this question.
Find a common multiplier then divide ex. 21/27 = 7/9