Average rate of change can be calculated by the following
Δy/Δx
Our Δx refers to displacement.
Δx = 4 - 64 = -60
Our Δy refers to our Δf(x)
To get our f(x) values, we must plug in 64 and 4 into the function f(x)
f(64) = 8√64 = 64
f(4) = 8√4 = 16
Δf(x) = 16 - 64 = -48
So our rate of change is -60/-48 = 0.8
I believe the answer should be positive 0.8, but that's my best estimate.