Let the portion of the job that Felix can complete in one hour=

Let the portion of the job that Max can complete in one hour=

As, rate * time = portion of job done
We can simplify this like:
Portion that Felix can do in 9 hours + portion that Max can do in 9 hrs = 1 job done





Hence, Felix can complete the whole job in 12 hours working alone.
Max can alone do the job in 3(12) = 36 hours.