Since you've shown no work, I'll guess that you're needing a nudge on how to get started:

Velocity is the derivative of position with respect to time, so differentiate.

Acceleration is the derivative of velocity with respect to time, so differentiate again.

To find when the velocity is zero, set the velocity function equal to zero, and solve for the time(s) t.

Take whichever techniques they have taught you for finding the maximum of a function, and apply these to the velocity function.