(Psst, this should have been posted in the Pre-Algebra/Algebra subforum.)
On the left side, both the 3 and x get squared. On the right side, the square-root and the power of 2 "cancel-out":
Then use the quadratic formula to solve for x.
EDIT: Beaten to it...
Of course, as I and others have said, this is still not the correct value of for your problem; I just wanted to point out another place you went wrong in the hope of being helpful.
For the love of (insert deity here), please remember this next time. I can't tell you how many times I've seen people make the same mistake. It's a pet peeve of mine. Arrrrgh!