The formula your looking for is Vf = Vi + A (T)
Vf = Velocity Final
Vi = Velocity Initial
A = Acceleration
T = Time
Our Initial Velocity is 15m/s. That’s our starting point.
Acceleration is -3m/s. It’s negative because we’re slowing down
We know that the cyclist comes to a complete stop, which means the Final Velocity is 0
In order to solve this with the equation. We need three of the four variables. We just don’t know time. So our setup is -
0 = 15-3(T)
Then you solve