Well in a right angle triangle, we know that angle C would be 90 degrees, we can check this by solving for X and seeing if the expression 2x will give us a value of 90 degrees.
So 2x = 90
X = 90/2
X = 45 degrees.
90 = 2x
90 = 2(45)
90 = 90.
Thus since the equation is true, X = 45 degrees, we can also solve for the remaining angles knowing what X is.