We know the equation of motion v = u+ at, where v is the final velocity, u is the initial velocity, a is the acceleration and t is the time taken.
Acceleration = - 9.81[tex]m/s^2[/tex] . Negative means acceleration is towards center of planet Earth.
Time taken = 0.50 seconds
Initial velocity = 18 m/s
v = u+at
v = 18-9.81*0.50
= 13.095 m/s
So velocity after 0.50 seconds is 13.095 m/s