What is the sum of the first eight terms of a geometric series whose first term is 3 and whose common ratio is .5 ?

Taking algebra two apex courses and need with these questions

Antonia and carla have an equivalant percentage of green marbles in their bags of marbles. antonia has 4 green marbles and 16 total matbles. if carla has 10 green marbles , how many total marbles does carla have

What is x3+3x2−16x−48 divided by x−1?