Answer
0.16 s.
Work Step by Step
Use the equation on page 541 for the voltage across a discharging capacitor.
$$V_C=V_oe^{-t/RC}$$
The capacitor voltage is to be 0.0025 of its initial value.
$$0.0025V_o=V_oe^{-t/RC}$$
$$0.0025 =e^{-t/RC}$$
$$ln(0.0025) = -t/RC$$
$$t=-RC\; ln(0.0025)=-(8.7\times10^{3}\Omega)(3.0\times10^{-6}F) ln(0.0025)=0.16s $$
