I keep trying to get this right but I keep falling short. Here's the problem: A hoodlum throws a stone vertically downward with an initial speed of 19.0 m/s from the roof of a building, 39.0 m above the ground. (a) How long does it take the stone to reach the ground? (b) What is the speed of the stone at impact? My professor has given us a number of formulas to use for motion problems and they are as follows: Vf = Final Velocity, Vi = Initial Velocity, a = acceleration, t = time, d = displacement First Formula: Vf^2 = Vi^2 + 2ad Second Formula: d = Vi(t) + (a/2)(t^2) Third Formula: Vf = Vi + at Which formula could I use to solve this problem? For part a based on what my textbook indicates it'd seem that the second formula will do it, but when I plug in the data that's given I end up with: 39m = 19t - 4.9t^2 But as you can see this doesn't give me a single variable. (Well it does, t, but I end up with a quadratic equation). Do I just solve the quadratic or am I doing something wrong? And if I'm doing something wrong, am I using the right formula of the 3 or do I need to use a different one? Please help! Thanks!