I believe you meant f(x) = x^2 - 3x - 10.
f(b) - f(a)
The ave. r. of c. formula is a. r. c = -----------------
b -a
(6)^2 - 3(6) - 10 - [(4)^2 - 3(4) - 10]
Here, a. r. c. = ------------------------------------------------------
6-4
36 - 18 -10 - 16 + 12 + 10
... which comes out to ---------------------------------------
2
a. r. c. = 7 for x^2 - 3x - 10 on the interval [4,6]
