It seems like the hint is asking for a single number that results from the combination of the two numbers given, which are 6 and -6. When you multiply 6 and -6, you get -36, but that does not align with the responses provided.
Considering the hint "think what two numbers make this happen" and looking at the choices, it could relate to the absolute value of those numbers. The absolute value of both 6 and -6 is 6, which makes sense if the answer is framed as one number.
So the answer would be 6.