He radius of a circular oil slick expands at a rate of 8 m/min. (a) How fast is the area of the oil slick increasing when the radius is 26 m?

If it's expanding 8m per minute, then you have to multiply 8 by 26.

1 minute = 8 m

2 minutes = 16 m

5 minutes = 40 m

10 minutes = 80 m

26 minutes = 208

8 * 26 = 208

So when it reaches 26 m in length, it'll be expanding 208m per minute.