Solving Projectile Problem: Initial Velocity from Cliff

[jysim]

Homework Statement

A cannon is fired at an angle of 30 degrees above the horizontal from a cliff that is 20m above a flat river bottom. What is the initial speed of the projectile if it is found to land 40m from the base of the cliff?

Homework Equations

g = 9.8ms^-2
Vi = initial velocity

The Attempt at a Solution

I know that Vi cos 30t = 40, but does the "t" represent the time taken for the entire projectile motion, or just the upper symmetrical parabolic trajectory? I'm stuck at Vi cos 30t = 40 because what I did after simply didn't make any sense at all.

I've been stuck on this question for days on end, please help me with it! Thanks! :D

 

[JHamm]

The $t$ represents the entire time of flight, from your equation you can actually solve for it. Now how about a second equation for the vertical part of the flight?