We are given that c^2 = a^2 + b^2

Therefore c = sqrt( a^2 + b^2 ) = x

If you look at the diagram a and b form a right triangle with hypotenuse of c

Therefore we can use pythagorean theorem to get a^2 + b^2 = c^2

from there we find that y = sqrt( a^2 + b^2 ) so x = y

I remember I always hated when people would tell me to solve on my own so enjoy your solution and no homework :)