A stone is thrown from a rooftop at time t = 0. Its position at time t is given by r(t) = (10t,−5t, 6.4−4.9t^2). The origin is at the base of the building, which is on flat ground. Distance is measured in meters and time in seconds.

(a) How high is the rooftop?

I know that the initial velocity r'(t) = 0 at t = 0. But I am unsure on how to solve for the height of the building using parametric equations.