Idon't understand what the 4 if for.

Multiplying an even number of negative numbers gives an answer that is choose from:

Jabari is power washing houses for a summer job. for every job, he charges an initial fee plus $30 for each hour of work. his total fee for a 4 -hour job, for instance, is $170 . jabari's total fee,f, for a single job is a function of the number,t, of hours it takes him to complete the job. write the function's formula.

What is the missing step in this proof ?