Finding coordinate on line on log-log graph with gradient in dB/decade

Question

I have an image of a plot showing peak-to-peak velocity (mm/s) against frequency (Hz) on a log-log scale. The curve is defined by an accompanying sentence to the effect of "The amplitude is $x$ between $a$ Hz and $b$ Hz. The curves decrease below $a$ Hz and above $b$ Hz at $n$ dB per decade".

I am not able to share the actual image, but it looks something like this: (Not to scale, of course)

    100 |
        |
v [mm/s]|
        |     a                  b
        |     ____________________
     10 |    /                    \
        |   /                      \
        |  /                        \
        | /                          . p
        |/__________________________________
        1        100      1000     10 000

              f [Hz]

How do I go about finding the y-coordinate of point p in mm/s? I know the x-coordinate of $p$ is 10000 Hz.

I am not sure how to convert the given gradient of dB per decade to mm/s per decade. I know the equation $\text{Gain}_\text{dB} = 10 \log_{10} (P/P_0)$, but I don't know what $P$ and $P_0$ are in my case.

Thanks!