Answer
$f = 480~Hz$
Work Step by Step
First we can find an expression for $f_1$.
In this situation:
$v_D = 0$
$v_S = 15~m/s$
We can find an expression for $f_1$:
$f_1 = f~\frac{v}{v-v_S}$
$f_1 = f~(\frac{340~m/s}{340~m/s-15~m/s})$
$f_1 = 1.046~f$
Then we can find an expression for $f_2$.
In this situation:
$v_D = 25~m/s$
$v_S = 15~m/s$
We can find an expression for $f_2$:
$f_2 = f~\frac{v+v_D}{v-v_S}$
$f_2 = f~(\frac{340~m/s+25~m/s}{340~m/s-15~m/s})$
$f_2 = 1.123~f$
We can find $f$:
$f_1+37~Hz = f_2$
$1.046~f+37~Hz = 1.123~f$
$37~Hz = 1.123~f-1.046~f$
$37~Hz = 0.077~f$
$f = \frac{37~Hz}{0.077}$
$f = 480~Hz$
