if a + b = 2 and a2 - b2 = -4, what is the value of a - b?

if a+b=2 and a2-b2=-4, what is the value of a-b?

a^2 - b^2 = -4

Factor this:
(a + b)(a - b) = -4

We know from above, (a +b) = 2, so substitute:
2(a - b) = -4

Divide each side by 2
(a - b) = -2