Given the position function
s(t) = 3 - cos(t )
differentiate once to get the velocity, and twice to get the acceleration:
v(t) = s'(t) = sin(t ) → v (4.3 s) ≈ -0.92
a(t) = s''(t) = v'(t) = cos(t ) → a (4.3 s) ≈ -0.40
The acceleration is negative, so the particle is slowing down.