A ball is tossed downward from the top of a 400 m skyscraper.?

a) d = vi*t + (1/2)at^2. The way the problem is phrased it isn't clear whether the initial velocity vi is zero or not. "Tossed downward" sounds like it's nonzero, in which case you don't know the value, in which case you don't have the information to solve the problem.
But assuming it's zero, then d = (1/2)at^2. Solve for t.

b) The energy is mgh (assuming vi = 0, so no initial energy). Set equal to (1/2)mv^2 and solve for v.