If the thrower takes 0.90 s to complete one revolution, starting from rest, what will be the speed of the discus at release?

Ω₀ = the initial angular velocity (from rest)
t = 0.9 s, time for a revolution
θ = 2π rad, the angular distance traveled

Let
α =  the angular acceleration
ω =  the final angular velocity

The angular rotation obeys the equation
(1/2)*(α rad/s²)*(0.9 s)² = (2π rad)
α = 15.514 rad/s²

The final angular velocity is
ω = (15.514 rad/s²)*(0.9 s) = 13.963 rad/s

If the thrower's arm is r meters long, the tangential velocity of release will be 
v = 13.963r m/s

13.963 rad/s

Hi! I'm a few days late since I just saw this question just now, but I'll answer anyway.

When an unbalanced force acts on a moving object, the velocity of the object will change. Change in velocity means a change in speed, direction, or both. For example, if you kick a soccer ball and it moves from one place to another, it means that unbalanced forces are acting upon it. The ball moves from one place to another after kicking it so the position changed.