With an isosceles triangle, isosceles means that two of the sides are equal, along with a 90° angle. You already know that one of the equal sides is 5√3. The value of the legs is always the hypotenuse {x}, multiplied by the √2. after doing this very confusing Pythagorean theorem problem, you should end up getting x=5.
P.S.: HopeIhelped... and if I didn't I can explain differently... :)