The equation: 3 + x = 7. will be false if any number except 4 is substituted for the variable. The value of the ... Example 2 Find the solution of each equation by inspection. a. x + 5 = 12 b. 4 · x = -20 ... 3x + 3 = x + 13, 3x = x + 10, 2x = 10, and x = 5.