Answer
$7\;\rm m/s$
Work Step by Step
The number of beats per second in this case is given by
$$ f_-=400-8=\bf 392\;\rm Hz$$
where $f_+$ is the frequency of the speaker on the ground and $f_-$ is the frequency of the speaker on the truck.
We also know that
$$f_-=\dfrac{v}{v+v_{car}}f$$
where $v_{}$ is the speed of sound wave.
$$v+v_{car} =v\dfrac{f}{f_-}$$
$$ v_{car} =v\dfrac{f}{f_-}-v$$
$$ v_{car} =v\left[\dfrac{f}{f_-}-1\right]$$
Plugging the known;
$$ v_{car} =343\left[\dfrac{400}{392}-1\right]$$
$$ v_{car} =\color{red}{\bf 7}\;\rm m/s$$
