Answer
$R = 12~\Omega$
Work Step by Step
We can find the equivalent resistance of the three resistors:
$R_{eq} = \frac{V}{I} = \frac{8~V}{2~A} = 4~\Omega$
We can find the value of $R$:
$\frac{1}{4~\Omega} = \frac{1}{10~\Omega}+\frac{1}{15~\Omega}+\frac{1}{R}$
$\frac{1}{R} = \frac{1}{4~\Omega} - \frac{1}{10~\Omega}-\frac{1}{15~\Omega}$
$\frac{1}{R} = \frac{15}{60~\Omega} - \frac{6}{60~\Omega}-\frac{4}{60~\Omega}$
$\frac{1}{R} = \frac{5}{60~\Omega}$
$R = \frac{60~\Omega}{5}$
$R = 12~\Omega$
