We'll just give the sides a variable, the hypotenuse is A, the height is B and the bottom length is C
A = 21
C = 15
This means the B = is 14.696938456699069 which is rounded to 15. But, this doesn't matter.
As long as we know the hypotenuse and one side we can find out that x is equal to about 45.584691402807024, round this to 46.
46 degrees is the answer.