Answer
$$R_2=16.0\ \Omega.$$
Work Step by Step
We have
$$\frac{1}{R_2}=\frac{1}{R}-\frac{1}{R_1}=\frac{1}{5.76\ \Omega}-\frac{1}{9\ \Omega}=0.174-0.111 =0.063$$
That is $$R_2=\dfrac{1}{0.063}=15.873\approx 16.0\ \Omega.$$
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