Start with the equation you know: area = pi r^2; most people use pi=3.14.
For this, you know the area, so 274 cm^2=3.14 r^2
Divide both sides of the equation by 3.14, so you have
You then have to take the square root of both sides of the equation to find r. I'll let you enter that into your calculator.
Whenever you have a math or physics problem, write down what you know (ie, what is given to you) Even if you can't solve for what the ultimate answer is, you can always find an equation that uses what you have been given, that will give you another value. You can then use that in another equation (or 2, or 3) to find the final answer. You didn't have to use more than one, this time--you just have to see what you have, and know that you know the basic equation. Then use it.