Answer
The horns emit a frequency of 82 Hz
Work Step by Step
We can find an expression for the frequency detected by the stationary driver from the horn in the moving car.
$f' = \frac{f}{(1-\frac{v_{source}}{v_{snd}})}$
$f' = \frac{f}{(1-\frac{18~m/s}{343~m/s}~)}$
$f' = 1.055~f$
The beat frequency is the difference between the two frequencies. We can find the frequency $f$.
$f'-f = 4.5~Hz$
$1.055f-f = 4.5~Hz$
$0.055f = 4.5~Hz$
$f = \frac{4.5~Hz}{0.055}$
$f = 82~Hz$
The horns emit a frequency of 82 Hz.