Answer: 9pi
Explanation:
Area of a circle is pi*r^2, there is not much of an explanation here, its more of a formula you must remember.
pi can be estimated to be 3.14 if you want to, Im not sure what your teacher prefers. You can plug it into a calculator and get a better approximation or you could write the pi symbol.
(3.14)*(3mm)^2 = 28.27mm^2 or simply 9pi