you can say an object slow down when the acceleration is negative which can be observed on a negative slope during the graph of acceleration, from a position equation lets derive 2 times to attain the acceleration equation
x(t) = 81/2t^2-3/4t^4
V(t) = 81X - 3t^3
A(t) = 81 - 9t^2
81 - 9t^2 < 0
t^2 > 9
-3 > t > 3