how do you arrive at that number?

The question assumes you know some things about the hypothetical FM transmitter being described.

In that transmitter, the oscillator generates the FM signal at a low frequency, which is then passed through frequency-multiplying stages to get to the output frequency. (This is a common design approach since the beginning of FM).

The important point is that not only is the carrier frequency multiplied, but so is the deviation caused by the modulating signal - and by the same factor (12 times).

73 de Jim, N2EY