und \( f(6) = 216 - 288 + 54 - 18 = -36 < 0 \), \( f(5) = 125 - 200 + 45 - 18 = -48 < 0 \), \( f(6.5) \approx 274.6 - 338 + 58.5 - 18 = -24.9 < 0 \), \( f(7) = 343 - 392 + 63 - 18 = 6 > 0 \).