4 times a number is 21 less than the square of that number. Find the positive solution.

the solution should be 7

Let x = your number.

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

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

0 = (x - 7) (x + 3). Set both terms equal to zero.

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

x = - 3. This is the negative solution.

(x - 7) = 0. Add 7 to both sides.

x = 7. This is the positive solution