How do I tell if my answer is reasonable?

Question: Assume that K is between J and L. If JK=20-x^2, KL=2-x, and JL=10 find x (make sure your answer is reasonable!)

I solved the problem and got -4, and 3 but the answer key says the answer is just -4, why doesn't   3 work?

Nevermind I found my answer! :)