How can i solve this does anyone knows?

A. C = 17h + 15
b. 83 = 17h + 15 
    -15           - 15  subtract 15 from both sides 
    68 = 17h
   
         17               divide both sides by 17
      4=h  
  They cleaned the house for 4 hours
c. C = 20x
to do this last part, you would put both equations into the calculator under y=, then look at the table to determine when the price is the same. According to the calculator, the price is the same at 5 hours 

I’m sorry but I don’t know number 3
1)15+17h
2)4 hours

Where are the functions?