6+7+8+...+(n+5) find the sum.

Ineed with number 17 i don’t get it at alll

Tom wants to order tickets online so that he and three of his friends can go to a water park the cost of the tickets is 16.00 per person there is also a 2.50 one-time service fee for ordering tickets online write an expression in term of n that represents the cost for n ordering tickets online

Y=(x+3)(x-4)(x+1)^2 sulutions? multiplicity?