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#
