The ratios of sides in a 30°-60°-90° triangle are such that the hypotenuse (AB) is double the length of the shortest side (AC). In your triangle, AC is the hypotenuse of similar triangle ACD, which has AD as its shortest side.
Then
(1/2)×(AD+54) = AC
(1/2)×AC = AD
Together, these tell you
AD = (1/4)(AD +54)
3×AD = 54
AD = 18
