The relevant formulas depend on what you do know, please advise.
If the acceleration you are missing is gravity than that is a known constant (9.8 m/sec^2). The formulas for projectile motion are:
y = yi + visin(A)t +.5gt^2
x = xi + vicos(A)t
with:
yi = equal to initial vertical position
xi = equal to initial horizontal position
vi = initial speed
A = angle from the horizon
t = time
g = gravitational acceleration (see above)
You can differentiate these equations to give the x' and y' components of velocity as a function of time. If you want me to do it you can write out the whole problem.
I hope this helps.