50
I attached a pic of my work.
so what I did was find the amount of the other sides of the triangle using their opposite congruent sides. aka, the bottom is 90, which means the top should also be 90. and 90-50 is 40, so the rest of that side (which is one side of the triangle) should be 40.
and then I used the pythagorean theorem, since it is a right triangle (indicated by the squares in the corners).
the theorem states that a^2 + b^2 = c^2
so just plug in the legs (a and b), and you should get the amount of segment AB (which is c)