Calculate the capacitor voltage for t  0 for the circuit below
For \$\small t\ge 0\$ you could use the general solution for 1st order transients, viz: \$\small v(t) = v(\infty)+(v(0)-v(\infty))e^{-t/\tau}\$, where \$\small\tau\$ is the effective time constant.

The voltage across \$\small C\$ for \$\small t&lt;0\$ is not \$\small 12 V\$, however. And you are correct in using Thevenin to replace the \$\small 4\Omega/ 2A \$ current source for \$\small t&lt;0\$