Finding Number of Factors of an integer - question?

Hi,

Can anyone please help me with the formula for finding the total no. of factors of an integer?

The usual way is prime factorization; then increasing the exponent of each prime factor by one; and multiplying the raised numbers.

E.g:

Q. Find the no. of factors of 90?

A.

90 = 2^1 x 3^2 x 5^1

So total factors = (1+1) x (2+1) x (1+1) = 2 x 3 x 2 = 12.

I have a test prep where solution actually doubles this answer saying that 12 are positive factors and there are as many negative factors.

So the total no. of factors should be 12 x 2 = 24.

But I researched a bit on the internet and almost everywhere they don't double the answer, i.e. they say answer is 12.

What is right on GMAT?

Thanks,
federite