# Thevenin equivalent impedance in laplace domain

I can't understand why $$Z_{th}$$ doesn't include $$3\ \Omega$$ resistance.

The $$\frac{4}{s}$$ is a current source so it's open when you calculate the Thévenin equivalent, so no current flows thorough the 3 Ohm resistor however $$\frac{12}{s}$$ is a voltage source so it's a short circuit while you calculate the equivalent impedance.