Answer
$36\,\Omega$
Work Step by Step
Capacitive reactance $X_{C}=\frac{1}{2\pi fC}$
Given $X_{C}=68\,\Omega = \frac{1}{2\pi \times(460\,Hz)\times C}$
$\implies C=\frac{1}{2\times \pi\times460\,Hz\times68\,\Omega}=5.1\times10^{-6}F$
When frequency $f=870\,Hz$,
$X_{C}=\frac{1}{2\times\pi\times870\,Hz\times5.1\times10^{-6}\,F}=36\,\Omega$