capacitor - How to calculate time constant for RC circuit with more than one resistor

The time constant in the solution is 0.5. (-2t implies tau = 0.5) I am wondering how they got this.

tau = RC

So I am wondering how to calculate the Rth. The capacitor is inbetween the two resistors in series, so would the Rth be

Rth = 6/(6+2)

But then tau = (6/8)*(1/3) = 0.25

if Rth is not calculated like this and is just = (6+2)

tau = 8*(1/3) which is not equal to 0.5

if Rth is also not calculated like this and is 2/(6+2)

tau = (2/8)*(1/3) which is not equal to 0.5

I know that I can get the Rth from the solution ie. since the solution says tau = 0.5, then the Rth must be 1.5ohms (since 1.5 * (1/3) = 0.5)

But I cannot see how the resistance at the terminals of the resistor is equivalent to 1.5ohms.

So my question is how to calculate Rth and how to calculate tau.