Average rate and instantaneous rate?

Hi everyone! :) I happen to be stumped on this question below.

A function y=f(x) and values of x0 and x1 are given.

y= x^3 , x0= 1 , x1= 2

a. Find the average rate of change of y with respect to x.

b. Find the instantaneous rate of change of y with respect to x at x0.

For a. I got 7 as my answer. I'm curious what everyone else gets. For b. I am stumped and know that you have to find a limit probably. Showing steps would be appreciated as I'd like to see how this is done.

(NOTE: We are not allowed to use derivatives yet, so it all has to be limits and stuff.)