An assortment of answers that students have submitted to this problem.

-16,-8,8,16
0
a+b
15
16384
-64 -9,9
3,5
(x+3)(x+5)
8
8x
x^2+8x+15
10
30
225
15376
16144
64
3,5,-3,-5,1,15,-1,-15
10,8
-7168
$$k>9$$
i dunno
sqrt15, -sqrt15
Qwertyuiopasdfghjklzxcvbnm
205520896

I do worry about the problems when the students have trouble agreeing on what category the answer should belong to.