$=\frac{1}{R_{t h}}=\frac{1}{3 R}+\frac{1}{R}=\frac{4}{3 R}=\frac{3 R}{4} R_{t h}=$ Thevenins resistance

So now adding the remaining resistance $\mathrm{R}$ with $R_{t h}$ in series we get the equivalent resistance

$R_{e q}=\frac{3 R}{4}+R=\frac{7 R}{4}$

And time constant $\tau=\mathrm{RC}$

$\tau=\frac{7}{4} \mathrm{RC}$