Need some help with data management

Hi all,

First of all I have gathered some data from my experiments witch looks something like this:

Now I do a linearization for my data using Matlab command:
y=feval(fit(a(l:o,1,1),a(l:o,2,1),'poly1'),a(l:o,1 ,1));
Results looks like this:

Now I do a interpolation using for example 5th order poly and get this:

I calculate the peak value and calculate -3dB value of peak value and get a line as you can see in the image:


Now my question is, what is the fastest and the best function to find the intersection x values of the 5th order polynomial and the line?

Thank you for your answers in advance,
Regards,
David.