Question

A concert promoter charges $65 per ticket for a concert. The promoters cost will be $5500 plus $4 per ticket.

Answer 1

Let the number of units sold be n

Revenue = 65*n

Cost function = 5500 +4*n

Profit function = Revenue - Cost

=65n -(5500+4n)

61n-5500
 

Answer 2

You would just plug in the formulas given to you following the hint it's:
r(n) - c(n)
which goes to
65n - (4n + 5500)
then we can simplify that to 
61n - 5500
Part 2:
61(850) - 5500
51850 - 5500 = 46350
this means if he sells 850 tickets he will make a net of 46350 dollars