In the right triangle, ac = bc = 2. what is ab?

Short x = 2*sqrt(2)
The Pythagorean Theorem states that 
a^2 + b^2 = c^2

Givens
a = 2
b = 2
c = ???

Sub and solve
2^2 + 2^2 = c^2
c^2 = 4 + 4
c^2 = 8                                  Take the sqrt of both sides of the equation
sqrt(c^2) = sqrt(8)                 Express 8 in it's prime factor form.
sqrt(c^2) = sqrt(2 * 2 * 2)

The rule about square roots is 
If you have a pair of prime factors under the root sign, you take out one of the prime factors and throw the other one away.

sqrt(c^2) = 2*sqr(2)

Answer x = 2*sqrt(2)