Square root both sides. The unreduced answer is root 28. Now. To reduce root 28, you need to break it down into factors and hope one of them is a perfect square.
4*7=28, and 4 is a square number. The square root of 4 is either positive or negative 2. Now that you've found that square, pull it out from under the square root symbol. Under the root symbol, you're left with 7 (not a perfect square).
Therefore, your answer is C