Recurrence Equation Question
To use any formula, you need to think about what it means. You can't just blindly apply formulas.

$C(n) = aC(n/b) + c$ (and, yes, I've changed the constants) means that the cost of running the algorithm on an input of size&nbsp;$n$ is $a$&nbsp;times the cost of running it on an input of size $n/b$, plus some constant overhead&nbsp;$c$.  A typical case of that would be an algorithm that chops its input into $b$&nbsp;equally sized parts and makes recursive calls on $a$ of those parts. Normally, you'd have $a=b$ (e.g., split the input into three and recurse on each part), which is why the formula you quoted has just two constants.  But you could imagine some sort of average-case analysis where you split into some number of parts and, on average, only need to look at half of them.