Negative x Negative = Positive ?????

Using the problem below describe why atleast theoretically we can prove:

negative x negative = positive.

8 x 7 = 56; is a fact easily provable in the real world (ex: 8 students each have 7 pencils, how many pencils do they have, 56)

Here is another way to write 8 x 7:

(10 - 3)(10 - 2)

Why does (-3)(-2) have to equal 6?