What is the sum of the first 7 terms of the series −4+8−16+32−... ?

Carla is saving money for a trip this summer. she already has some money in her savings account and will add the same amount to her account each week. at the end of 2? weeks, carla has? $130. at the end of 8? weeks, she has? $280. write a linear function in the form y=mx+b to represent the amount of money m that carla has saved after w weeks.