A car's horn emits sound having a dominant frequency of 2400 Hz ....

The speed of the car (source) is $v_s = 18.0 \text{ km/h} = 18.0 \times \dfrac{5}{18} \text{ m/s} = 5 \text{ m/s}$.

The observer is stationary, so $v_o = 0$.

Using the Doppler effect formula for a source approaching a stationary observer:

f' = f \left(\dfrac{v}{v - v_s} \right)

Substituting the given values:

$f' = 2400 \left( \dfrac{340}{340 - 5} \right)$

f' = 2400 \left(\dfrac{340}{335} \right)

$f' \approx 2435.8 \text{ Hz}$

Rounding to the nearest integer, the apparent frequency is $2436 \text{ Hz}.$