g(x) = 6(4)x, Section A is from x = 0 to x = 1. Your "6(4)x" needs to be written as
g(x) = 6(4)^x, where the " ^ " indicates exponentiation.
The average rate of change of g(x) from x= 0 to x=1 is:
6(4)^1 - 6(4)^0 24 - 6
a.r.c. = ----------------------- = ----------- = 18 (answer)
1-0 1
Now do the same thing for x = 1 to x = 2.