Given the position function s(t), you can get the acceleration function by differentiating s twice:
velocity = s'(t) = -5 sin(t ) + 3 cos(3t )
acceleration = s''(t) = -5 cos(t ) - 9 sin(3t )
Then when t = π, the particle's acceleration is
s''(π) = -5 cos(π) - 9 sin(3π)
… = -5 • (-1) - 9 • 0 = 5
