8 minus some number divided by 3 euals 6 what is the number

Hillel is juggling flaming torches to raise money for charity. his initial appearance raises $500\$500 $500 , and he raises $15\$15 $15 for each minute of juggling performance. the amount rr r of money hillel raises is a function of tt t , the length of his performance in minutes. write the function's formula.

Any is very appreciated! (picture is provided below)not sure how to do this at all won't lie

What is the sum of the first 8 terms of the geometric series