The area of a circle is 528 feet. find the diameter.

The area of a circle can be written as  $$Area=\pi\frac{D^2}{4}$$  where D is the diameter.

$$$528=\pi\frac{D^2}{4}$\\ Multiply both sides by $\frac{4}{\pi}$\\ $$ D^2=\frac{4*528}{\pi} $$\\ take the square root of both sides\\ $$ D=\sqrt{\frac{4*528}{\pi}}

$${\mathtt{D}} = {\sqrt{{\frac{{\mathtt{4}}{\mathtt{\,\times\,}}{\mathtt{528}}}{{\mathtt{\pi}}}}}} \Rightarrow {\mathtt{D}} = {\mathtt{25.928\: \!179\: \!257\: \!714\: \!298}}$$   feet

or D ≈ 25.93feet

 Incidentally, the area would be in square feet, not just feet!