factoring question

Question

A smart-phone is thrown upwards from the top of a 336-foot building with an initial velocity of 64 feet per second. The height h of the smart-phone after t seconds is given by the quadratic equation  h = −16t² +64t +336

When will the smart-phone hit the ground?

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Answer 1

We need to solve the equation for t when h=o. Because it is a quadratic equation, we will get 2 values for t, but because time cannot be negative we will use only the positive value.

                                                    h = -16t² + 64t + 336

                                                    h = -16(t² - 4t - 21)

                                                    h = -16(t - 7)(t + 3)

                                                    0 = -16(t - 7)(t + 3)

                                                    t = 7 or -3

Again, because time cannot be negative, we can safely say that the time would be 7 seconds.