A gas has a volume of 350 mL at 45°C. If the volume changes to 400 mL, what is the new temperature?
The correct equation to use with these kinds of questions is the Combined Gas Law.
This equation, though, is a little unnecessary at times because of the possibility of extra constants.
In this case you have a pressure constant which enables us to shorten your longer equation down to Charles' Law because pressure remains the same and doesn't affect the calculation.
The next step is to isolate the variable that you want, which is, in this case,
Now it is time to replace your known variables with numbers