Given 3 3 = 5 and 6 4 = 20, what is the rule for a b?

The problem defines a custom mathematical operator $\Psi$ using two examples:

1. $3 \Psi 3 = 5$
2. $6 \Psi 4 = 20$

We need to find a rule $a \Psi b = f(a, b)$ that satisfies both examples.

Let's test a common pattern for such problems: $a \Psi b = (a \times b) - C$.
Using the first example:
$3 \Psi 3 = (3 \times 3) - C = 5$
$9 - C = 5$
$C = 9 - 5$
$C = 4$
So, the rule could be $a \Psi b = (a \times b