pH is defined as the -log of the molar concentration of hydrogen ions in solution:
and that's what we need to find in order to solve the problem.
The common link between the information given and the information needed to solve the problem is the number of moles of hydrogen ions (or hydronium ions).
We are told that we have 40.4mL of 0.100M ...
The solution defines pH as well as provides a step-by-step guide to understanding and answering the problem. The attachment if a formatted version of the answer.