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

set up the equation 4x + 21 = x^2. getting everything on one side so you have x^2-4x-21 = 0. find two numbers that multiply to - 21 and add to - 4, which are 3 and - 7. substitute in with them as x to get x^2+3x-7x+21 = 0. factor out the x and the 7 to get x (x+3) - 7 (x+3) = 21. so x = - 3 and 7. the positive solution would be 7.