Write down what you know. First here are the numbers we know:
V = 355
Here is the formula we know:
V = πr²h
We can substitute in the numbers for the letters we know (I'm putting parentheses - brackets - around them so you can see what is happening):
 = [3.14] r² [12.2]
Now I have an equation (it has an equal sign, so it is an equation) with just one letter in it. So I want to isolate that on one side of the equal sign.
[3.14] r² [12.2] = 355
Divide by [3.14] and [12.2] on both sides so that those factors on the left cancel out and r² is left by itself:
r² = 355 ÷ [12.2] ÷3.14
r² = about 9.267
We want to know the radius r but we have r² instead. So we need to take the square root of both sides now:
√(r²) = √(9.267)
r = √(9.267)
r = about 3.04 cm
I hope this step by step explanation helps!
PS: It is important for you to recognize in this question that 1 ml = 1 cm³. If the units were in fluid ounces and inches, you'd definitely have more work to do. Metric makes things quite simple.
Math Central is supported by the University of Regina and The Pacific Institute for the Mathematical Sciences.