Eric C. answered 09/27/16
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Hi Winston.
The average rate of change of a function f(x) between a and b is given by:
(f(b) - f(a)) / (b - a)
Here, your f(x) is 6 - x^2
a = 3
b = 6
f(b) = 6 - 6^2 = -30
f(a) = 6 - 3^2 = -3
So
(f(b) - f(a)) / (b - a)
= (-30 - -3) / (6 - 3)
= -27 / 3
= -9
Your average rate of change is -9.