Start by remembering that the square root symbol gives the non-negative root. The equation says that √(x - 4) is 3. To remove the square root, square both sides: x - 4 = 3² = 9. Now isolate x by moving the constant to the other side, so x = 9 + 4 = 13. Always check radical equations: substituting x = 13 makes the expression inside the root equal to 9, and √9 = 3. So the correct answer is 13.