the average rate of change formula is
f(b) - f(a)
arc = ----------------
b-a
Here, a= -10 and b = 10
(10)^2 + 9(10) + 18 - [ (-10)^2 + 9(-10) + 18]
Here arc = --------------------------------------------------------------
10 - (-10)
10^2 and -(-10)^2 cancel. 18 - 18 cancel.
90 - (-90) 180
This leaves us with arc = ----------------- = ------------- = 9 (answer)
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