Chapter 1 - Equations and Inequalities - 1.6 Other Types of Equations and Applications - 1.6 Exercises - Page 149: 111

$\{ 16\}$

Step 1. Let $u=\sqrt x$, we have $u^2-u-12=0$ Step 2. Factor and solve the equation: $(u+3)(u-4)=0$, thus $u=-3, 4$ Step 3. For $u=-3$, we have $\sqrt x=-3$, thus $x=9$ Step 4. For $u=4$, we have $\sqrt x=4$, thus $x=16$ Step 5. Check answers in the original equation, only $\{ 16\}$ fits.