Why For a sine wave, the RMS (effective) value is the peak value divided by the square root of 2, or equivalently peak times 0.707. So 17 V peak x 0.707 = about 12 V RMS. RMS matters because it is the AC voltage that produces the same heating power as that many volts of DC.
Watch out The 24 volt answer comes from multiplying by 1.414 instead of dividing, which gives peak from RMS, not the other way around. Half the peak, 8.5 V, is not a meaningful sine wave quantity here, and 34 V is the peak-to-peak value.
RMS = peak x 0.707 (smaller). Peak = RMS x 1.414 (bigger). RMS is always less than peak.