Define the frequency of the car as $f_0$, the heard frequency when it's approaching as $f1$ and the heard frequency when it's departing as $f_2$
Write the Doppler formula for both $f_1$ and $f_2$ using a receiver velocity of 0 (since the policeman is stationary with respect to the air) and a car velocity of $v_s$. Pay attention to the sign of the source velocity.
Write the ratio of both formulas and equal it to a major third. $f_0$ should drop out and you are left with a single equation for $v_s$