Diane was standing on a roof and threw a weight with a string attached to a nearby roof. The height of the weight above the ground is modelled by the function h(t)= -5t^2 + 10t + 18, where h(t) is height in metres and t is the time in seconds after the throw. Write the function in vertex form.

b) What is the maximum height of the weight?

c) After how many seconds does the weight reach the ground? Round your answer to the nearest tenth.