Cos2(3pi/4) find the exact value

Guys i really need or i have to stay late up night

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

3.4a=57.8 what is the the value of a