A right isosceles triangle can only ever have two equal sides, since he sum of the three angles of any triangle is equal to 180 degrees, do the quick math of 180 - 90 = 90, then 90/2 = 45. Both remaining angles of the triangle are 45 degrees, including x, and when you add all angles together 90 + 45 + 45 it equals 180 degrees.
