SAT Mathematics Practice Test 7

Factoring out the coefficient $\frac{1}{3}$ the given expression can be rewritten as $\frac{1}{3}\left({\mathit{x}}^2-6\right)$. The expression ${\mathit{x}}^2-6$ can be approached as a difference of squares and rewritten as $\left(\mathit{x}-\sqrt{6}\right)\left(\mathit{x}+\sqrt{6}\right)$. Therefore, k must be $\sqrt{6}$