if det(A)=1 what is det(-2A) ? shouldn't it be -2, or am I totally lost on this one?
I believe this is what you need to know:
If a is a constant and A an n×n square matrix, then |aA|=a^n|A|.
Thus, . So it depends on the size of your matrix - if it's, say, a 2 x 2 matrix, then
Make sure to use the size of your matrix for n.
(This is result 11 here: Determinant -- from Wolfram MathWorld. I actually don't recall this particular result, so there may be another method, particularly if you don't know the size of the matrix.)