Its a right angled triangle. You have C and have B all you need to find is A. usually for a question like this you would use the Pythagorean theorem which is A2+B2=C2 but since we already have c we will use C2-B2=A2 Now we fill in the values. (21)2-(9)2=A2 441-81=360^2 so now with the 360^2 we have to squareroot it so the square root of 360 is 18.973f