Answer
$150$ Hz, $450$ Hz
Work Step by Step
Remember that $f_n=nf_1$, so all frequencies must be equal to a whole number times the first harmonic. 600 Hz, 750 Hz, and 900 Hz hint that the distance between harmonics is 150 Hz and the fundamental frequency may be 150 Hz. In this case, the two missing values would be $150(1)=150$ Hz and $150(3)=450$ Hz.
