Cyclone Efficiency

I have a problem where I've been asked to find the cut diameter for a set of cyclones.

The equation I plan to use is
Dpc = (ln2/M)^(n+1)

I know n=0.945 but need to calculate M in order to find Dpc.

The grade efficiency for a particle size Dpi = 3 x 10^-6m is 0.6536.

So, I should be able to calculate M from the equation

efficiency = 1-exp(-M x Dpi^N)
where N = 1/n+1

therefore
0.6536 = 1-exp(-M x 3 x 10^-6^0.514)

However, I can't see how to calculate this - I know it seems a really basic question but I'm stuck! I could do with some guidance on how to calculate M by getting around the exp function.