To find the roots (or zeros) of a quadratic function like this one, you can use the quadratic formula or, if it's possible, use factoring. This quadratic factors pretty easily so that's what I'll use.
Factor the right side:
The Zero Product Property tells us that this or any product can be zero only if one or more of the factors is zero. This tells us that:
Solving each of these we get:
The largest root (or zero) of the equation, therefore, is 22.
(Note: If you choose to use the quadratic formula, you will get the same answers (if you use it correctly).)