The area of a circle is \(\pi\) times the squared radius of the circle,
\(A=\pi r^2\)
You're given the diameter, which is twice the length of the radius,
\(d=2r\implies r=\dfrac d2\)
Plugging this into the area formula gives
\(A=\pi\left(\dfrac d2\right)^2=\pi\dfrac{d^2}4\)
The radius is 28 m, so the area is
\(A=\pi\dfrac{(28\text{ m})^2}4=196\pi\approx615.75\text{ m}^2\)
