Hello....
v = 18 km/h = 18 x 1000/3600 = 5 m/s
ΔEc = 1/2 x m x v² - 0 (cause the bus stops) = mv²/2 = 25m/2 with m in kg
P = ΔEc/Δt with Δt = 2,5 s
To go further, we need the mass of the bus (m) or the length it took to stop the bus.
Supposing m = 10000 kg for instance
Then ΔEc = 25x10000/2 = 125000 J
And P = 125000/2,5 = 50000 W