Answer
$f_1=150$ Hz
Work Step by Step
A formula relating frequencies and the harmonic number is $$f_n=nf_1$$ where $f_1$ is the harmonic frequency. Therefore, substituting $n$ for 5, substituting $f_5$ for $750$ Hz, and solving for $f_1$ yields $$f_1=\frac{f_5}{5}=\frac{750Hz}{5}=150Hz$$
