Strange question and one that I can't work out either.
If the question meant to say VHF, then you'd use the formula:
Line of sight dist (NM) = 1.06*Sq Rt of Transmitter Ht (Ft) + 1.06 * Sq Rt Of Receiver Ht (Ft)
= 134.08 + 11.61......145.69nm.
I've no idea how they got 171.8nm.
As for HF......who knows. Sky waves + Ionosphere + Tx power + .......... Too many variables I think.