If the goal is to find a separate fit for every frequency given, you could also use "line" to do so.
Here for 300kHz:

"B300" is the very same which you called "b". But you would get a slightly different value when you do the same for the other frequencies.
10^A300 is your a*f^c with f=300.
But I guess that you are looking for constant values a, b and c which are valid for any frequency.
To find these values I would suggest using a solve block with "minerr":

Thereafter you may define a function P which is only dependent on B and f and because with Prime we are using something similar to Mathcad, it may be a good idea to add correct units

Now you can use this function with any input value you like

and of course also for plotting with different frequencies

And as its a function with two arguments, you may also create a 3D-Plot if you like

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