If the equation of motion of a particle is given by s = A cos(ωt + δ),

Question

the particle is said to undergo simple harmonic motion.
(a) Find the velocity of the particle at time t.
s'(t) = - A ω sin(ωt + δ)

I figured out how to do part a, but I don't know how to solve for part b.

(b) When is the velocity 0? (Use n as the arbitrary integer.)
t = __________