When 21 is subtracted from the square of a number, the result is 4 times the number. Find the positive solution.

Let x = your number.

x^2 - 21 = 4x. Subtract 4x from both sides.

x^2 - 4x - 21 = 0. Factor.

(x - 7) (x + 3) = 0. Set each term to 0.

x + 3 = 0. Subtract 3 from both sides.

x = - 3. This is the negative solution.

Set the other term equal to zero.

x - 7 = 0. Add 7 to each side.

x = 7. This is the positive solution.