Action Script Recursion

trying to solve this Question..

any tips on where to go with it?

Use the following function definition:

function countChar(str:String, ch:String, n:Number)
To count the number of occurrences of ‘ch’ in ‘str’
For example:
countChar(“aabba”,”a”,0) = 3
countChar(“asdf”,”z”,0) = 0
Hint: need to use string.substring() method