The time constant of an RC circuit is given by the product between the resistance and the capacitance of the circuit:
[tex]\tau = RC[/tex]
where R is the resistance and C the capacitance.
For the circuit in our problem,
[tex]R=2M\Omega = 2 \cdot 10^6 \Omega[/tex]
and
[tex]C=140 \mu F = 140 \cdot 10^{-6}F[/tex]
Therefore, the time constant of this circuit is
[tex]\tau =RC=(2 \cdot 10^{6} \Omega)(140 \cdot 10^{-6} F)=280 s[/tex]
