Answer
$1.0\times10^6\Omega$
Work Step by Step
Equation 19–7c tells us that the time constant equals the resistance multiplied by the capacitance, $\tau=RC$.
$$R=\frac{\tau}{C}=\frac{3.0s}{3.0\times10^{-6}F}=1.0\times10^6\Omega$$
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