a) What is the maximum altitude the rocket reaches?

b) What is the rocket's acceleration 8.5 seconds after launch?

c) What is the total time of flight?

d) What is the rocket's total displacment during the entire flight?

I'm only stuck on letter a right now, but if I need further help, I'll let you guys know.

For part a, what I tried doing is using this equation,

x-x0=vo(t)+(1/2)a(t)^2

where x0=0

vo=0

a=5 m/sec^2

t=7

I get x= 122.5 meters. Computer won't take it, so I'm scratching my head to no end. Any ideas to help me get started?