f(2) = -(2)^2 + 12 = -4 + 12 = 8

f(4) = -(4)^2 + 12 = -16 + 12 = -4

2. Calculate the average rate of change by using the formula:
Average Rate of Change = (f(4) - f(2)) / (4 - 2)

3. Substitute the values of f(4) = -4 and f(2) = 8 into the formula:
Average Rate of Change = (-4 - 8) / (4 - 2)
Average Rate of Change = -12 / 2
Average Rate of Change = -6

Therefore, the average rate of change of the function f(x) = -x^2 + 12 between x = 2 and x = 4 is -6.