Looks like you have chatAt(i) instead of charAt(i). Also, instead of char[k], try vowels[k].

This is my rewrite of the intro sentences:

You have a string with N numbers, starting with the third number each number is the result of an operation performed using the previous two numbers.

Write a function which returns a string of the operations in order and separated by a comma and a space (e.g. "subtraction, subtraction, addition").

Allowable operations are: