What is the positive solution of x squared - 4x - 21 = 0?

Correct answer to question 148 (Mathematics Knowledge): 7

Method

Find two numbers multiplying to -21 and adding to -4: those are -7 and 3, so the equation factors as (x - 7)(x + 3) = 0. The solutions are 7 and -3, and the positive one is 7.