To find the instantaneous rate of growth at a specific time, we need to take the derivative of the growth function with respect to time.

The derivative of F(t) = 81t^3 + 90 is given by:

F’(t) = 3 * 81t^2

To find the instantaneous rate of growth at t = 3 minutes, we substitute t = 3 into the derivative:

F’(3) = 3 * 81(3)^2 = 3 * 81 * 9 = 2187

Therefore, the instantaneous rate of growth in 3 minutes is 2187.