You could move the constant term to the other side by adding 6 to both sides, then take the square root. Just remember to consider both the positive and negative roots.
You could use the quadratic formula with , , and . Remember the quadratic formula is:
You could use the fact that and factor the quadratic using the difference of two squares factorization. Then apply the Zero Product Rule. Personnally, I like this method best because it won't allow you to forget to consider the negative root and it isn't as messy as the quadratic formula.