1/3 *3.14*r2*20=18149.2

1/3 *3.14*r2*20=18149.2

It appears that we have the volume of a cone with a height of 20 and we'd like to find its radius.

OK....let's start by multiplying both sides by 3 to "clear" the fraction on the left...this gives

3.14*r2*20 = 54447.6   Now, divide both sides by the "20" and the "3.14" and we have

r^2 = (54447.6)/(20*3.14)

r^2 = 867

Now, just take the square root of both sides and you're done.......you'll have r = (something)

I think that this is your question - you should have put an ^ (hat) between the r and the 2. ^ is the symbol used for power of when you don't know how to put it in the correct position. 

$$\frac{1}{3}\times3.14\times r^2\times20=18149.2$$

I'm sure CPhill's answer is perfect although I will admit i haven't thoroughly checked it.