Serena is writing an expression that is equivalent to 12ab – 6b, but she has not completed it. __?__ (2 a minus 1)

What is missing from the expression?
6b
6ab
6
ab

Ben frank is an electrician who installs lighting fixtures. he charges (in dollars) his clients 15h+40f15h+40f where hh is the number of hours worked and ff is the number of fixtures installed. how much does ben frank charge when he works 77 hours and installs 99 fixtures?