Unsure of how to find the equivalent resistance of the following circuit due to the input resistance of the LM386 amp
I think as a first approach I would consider the \$50\;k\Omega\$ input impedance constant coming in parallel with \$R_3\$. Your circuit shows a low-pass filter buffered then driving a high-pass filter. The transfer function from source \$B_1\$ to \$R_3\$ is thus:

\$H(s)=\frac{1}{1+sR_4C_3}\frac{sC_2(R_3||R_{in386})}{1+s(R_2+R_3||R_{in386})C_2}\$

The calculations are given below

You can rearrange this expression in a low-entropy format as shown here

How to find Vout for the following band pass filter