Quadratic formula help

Question

I'm having a hard time trying to figure this problem out, can anybody point me to the right direction or help?

A smart-phone is thrown upwards from the top of a 672-foot building with an initial velocity of 16 feet per second. The height h of the smart-phone after t seconds is given by the quadratic equation h=−16t^2+16t+672. When will the smart-phone hit the ground?

Answer 1

Ground is at h = 0, so if you solve 0 = -16t^2 + 16t + 672, you will find t when the phone hits the ground.

You'll actually get two answers. Ignore the negative answer because that answer is back in time.: )