How to find time using initial velocity and acceleration?

Use v = u + a*t   where v is final velocity (0 m/s here), u is initial velocity (4 m/s), a is acceleration (-9.81 m/s²) and t is time.

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You can't. You need at least three known variables.

You have initial velocity and acceleration, so you will need either displacement or final velocity to find time.

I am given the initial velocity of 4m/s and acceleration of 9.81 (gravitational) and told to work out how long it takes to reach the maximum height