First, here is the formula for finding circumference using the RADIUS:
C (circumference) = 2 x pi (3.14) x r (radius)
using the DIAMETER:
C = pi (3.14) x d (diameter)
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Using the above formula, just plug in the numbers!!
C = 2 x (3.14) x (8)
C = 50.24
50.24 is your answer
if you wanted to use diameter, you should know that diameter is just the radius times 2!