in the right triangle below the length of ac is 30 . in a 45-45 -90 triangle

Where is "the right triangle below" ??.

I would assume ac is the hypotenuse , then ab would be one of the legs (both legs are the same length)

Pythagorean theorem says     leg1^2 + leg2^2  = hyp^2

      since  leg1 = leg2  this becomes

                2 (leg1)^2 = 30^2

                   (leg1)^2 = 450

                    leg1=ab = sqrt450=21.21 units