How to find the equivalent resistances for each filter so that the cutoff frequencies can be found?
1) Start with the hypothesis that those two poles, the low pass and high pass ones, are widely frequncy separated so they virtually do not interact at all. Note that so far this is only an hypothesis and MUST be confirmed later upon workout completed 

2) In detail this means:


the high cut capacitor C3 is virtually open circuit while (at low
frequencies) C2 is doing its job.
Conversely, C2 is virtually short circuit when C3 comes in (high
frequencies).


in short \$f_\text{L}\ll f_\text{H}\$

3) So it's now easy to work out C2 and C3 seen resistances

\$ R_\text{C2}=R_2+(\;R_3 \| (R_4+R_\text{in LM386})\;)\approx 75\,\text{k}\Omega\$

and

\$R_\text{C3}=R_\text{in LM386}\|(\;R_4+(R_2\|R_3)\;)\approx 27\,\text{k}\Omega\$

4) and associated pole frequencies

\$ f_\text{L}=\frac{1}{2\pi R_\text{C2}C2}\approx 21\,\text{Hz}\$

and 

\$ f_\text{H}=\frac{1}{2\pi R_\text{C3}C3}\approx 59\,\text{kHz}\$

5) It's now time to verify the start off hypothesis \$f_\text{L}\ll f_\text{H}\$, clear enough it is widely fulfilled.

Note: This applies to first OP post. As he keeps on changing his question what I wrote may as well be not true any longer.