Explanation
Probability distribution
$x = 0$, $p = \frac{^7C_2}{^{10}C_2} = \frac{42}{90}$
$x = 1$, $p = \frac{^7C_1 \times ^3C_1}{^{10}C_2} = \frac{42}{90}$
$x = 2$, $p = \frac{^3C_2}{^{10}C_2} = \frac{6}{90}$
Now,
$\mu = \sum x_i p_i = \frac{42}{90} + \frac{12}{90} = \frac{54}{90}$
$\sigma^2 = \sum p_i x_i^2 - \mu^2 = \frac{42}{90} + \frac{24}{90} - (\frac{54}{90})^2$
$\Rightarrow \frac{66}{90} - (\frac{54}{90})^2$
$\sigma^2 \Rightarrow \frac{28}{75}$