Explanation:
strange, you solved much more complicated questions. this here is just to calculate a formula with the given parameters.
the only tricky thing, maybe : the % are to be used as decimal (4.5% = 0.045).
so, the balance after 15 years (compounded quarterly, so n = 4) is
3500×(1 + 0.045/4)^(4×15) = 3500×(1.01125)⁶⁰ =
= 3500 × 1.956645179... = 6,848.258127... ≈
≈ $6,848.26