This is just a simple substitution.

If we say the A note in the 6th octave is the "0 note", then x = 0.

Since there are 12 notes in the octave,

A in the 7th octave gives x = 12

A in the 8th octave gives x = 24

A in the 9th octave gives x = 36.

The list of notes in each octave is...

Bb, B, C, Db, D, Eb, E, F, Gb, G, Ab, A.

So Eb is 6 higher than A.

Which means Eb in the 9th octave gives x = 42.

Thus the frequency of Eb in the 9th octave is

.