The formula for this is

. If the diameter is 8 the radius is 4, so filling in accordingly we have

and

which is

and when you multiply in 3.14 you get that A = 401.92 or B above.
Step-by-step explanation:
<R = 180-<P-<Q
<R= 180-60-75
<R = 45°.
hope this helps you