Recall that the acceleration due to gravity is -9.81 m/s^2. The units are vital here, as the velocity v given in the problem is 80 (mm)/s, which needs to be converted to standard meters before continuing with the problem:
(80 cancel(mm))/s * (1 m)/(1000 cancel(mm)) = (80 m)/(1000 s) = 2/25 m/s = 0.08 m/s
Now we can proceed. For velocity problems, a common useful formula is:
v_f = v_i + at
In this formula, v_i represents the initial velocity, a represents acceleration, t represents time, and v_f represents the final velocity. For our problem:
{ (v_i = 0.08 m/s) , (a = -9.81 m/s^2) , (t = 3 s) :}
So:
v_f = (0.08 m/s) + (-9.81 m/s^2)(3 s)
= 0.08 m/s - 29.43 m/s
= -29.35 m/s