Deriving/Applying the Geometric Series Formula Practice Test

Verify the formula by computing both ways: Which value equals $$S_4 = 3 + 6 + 12 + 24$$ and also equals $\dfrac{3(1-2^4)}{1-2}$?

A phone trade-in promotion gives you $\$$400$ today, but if you wait, the offer decreases by $15%$ each week. If you wait 6 weeks, the total value of receiving the offer each week (hypothetically adding all weekly offers) is modeled by the finite geometric series $$400 + 400(0.85) + 400(0.85)^2 + \cdots + 400(0.85)^5.$$ What is the exact sum in geometric-series form?