An oil slick is expanding as a circle. the radius of the circle is increasing at the rate of 20 meters per day. express the area of the circle as a function of d, the number of days elapsed.

The circle expands it radius 20m each day. From this description you can get this equation:
r=0+ 20*d
r=20d

The oil slick expands as a circle and the formula for circle area is  r^2.  If you insert the first equation into this equation you will got:
circle area =  r^2
circle area =  (20d)^2
circle area =  400*d^2