Since we only know the diameter and the formulas use radius, we have to divide the diameter by two in both formulas. The trick is to remember the order of operations.
A= πr^2
A=(3.14)(5/2)^2
A=(3.14)(2.5)^2
A=(3.14)(6.25)
A=19.625 feet squared
Circumference=2πr
C=2(3.14)(5/2)
C=2(3.14)(2.5)
C=15.7 feet
Hope this helps! :)