Finding equivalent resistance for time constant in R-C circuit
This is slightly tricky given the presence of the controlled source.

While I won't work the problem for you, I will tell you that you can combine the controlled current source and the two rightmost resistors into one equivalent resistance \$R_{EQ}\$ which is then in parallel with the \$50k\Omega\$ resistor.

Here's a hint:  the voltage across the two rightmost resistors (and thus, the current source) is just \$(15k\Omega + 25k\Omega)i(t) \$

But the current through the top wire connecting the capacitor to the current source / resistors is just \$0.75 i(t)\$.  Thus, the equivalent resistance to the right of the capacitor is... 







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