Answer:
84
Step-by-step explanation:
f(1)=4^1 - 1 = 3
f(4) = 4^4 - 1 = 255
rate of change = 
(255 - 3) / (4 - 1) = 252 / 3 = 84
Abcdefghijklmnopqrstuvwxyz
ok
b to a
goes back 1 letter
then b to d
skips 3 letters forward
then d to e
1 forward
e to h
skips 3 forward
h to g
goes 1 back
g to k
goes 3 forward
pattern seems to be
1back, 3 forward, 1 forward, 3 forward, repeat
so we are at 3 forward after than 1 back, so the next one is 1 forward
1 forward from k is l
the next letter is L
The surface area will increase by a factor of 49/16
The answer is 18^8 because u add exponents