The formula to use could be either max likely range (NM) = 1.23√H1 + 1.23√H2 where H1 and H2 are the heights of the transmitter and receiver in feet or 12√(Height of the aircraft as a flight level). The second formula assumes a sea level transmitter which makes the first formula is more appropriate in this case. So,

113NM = 1.23√169 + 1.23√H2

Therefore        113 ÷ 1.23 = (√169 + √H2)

(113 ÷ 1.23)  - √169 = √H2

And                  H2 = [(113 ÷ 1.23) - √169]2 = 6220ft or approximately FL60.

Most pilots, being fundamentally lazy, would avoid this mathematical exercise by entering each of the four multi-choice answers into the formula until they found an answer that gave a range of 113NM. Either method is acceptable.