when i want to calculate time constant from this circuitshou

when i want to calculate time constant from this circuit,shouldn\'t i use test source to get resistor when there is a dependant souce ?
i can\'t directly open current source and short voltage source right?

so why this is right? please help me

Solution

Time constant does no depend on any source.its depend only on components. That\'s why we short the all type of source . if we calculate the current of voltage then it matter in calculation ..