​ please explain how to do this and the answer?

The shaded sector is  60 degrees out of the ENTIRE circle's 360 degrees

Find the area of the ENTIRE circle by using   Area = pi r²

     then multiply by   60 / 360   to get the answer in ft²

There are 360 degrees in a whole circle, you have 60 degrees.  SO you have   $\frac{60}{360}$   of a circel

The area of a whole cirlce is  $A=\pi r^2$

If this was the whole circle the area would be    $\pi* 30^2 = 900\pi \;feet\;squared$

SO the area of just the sector will be     $\frac{60}{360}*\pi*900 \;\;feet squared$

and you can do the calculation.