Rewrite the equation by completing the square. x^2-20x+100 = 0
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You find this value by factoring. Factoring finds the zeros of the function. When the ball hits the ground, it has 0 height. Therefore, when the function goes through the x axis, it has a y value of 0. That means that when y = 0, the ball has hit the ground. Use the quadratic formula to factor 
. Notice I rearranged the terms. When you factor that you get zeros of -3.19 and 2.19. The two things in math that will never EVER be negative are time and distance/length. Therefore, that tells us that the ball hits the ground 2.19 seconds after it was thrown.