TeX source:

\sigma _{M_1 - M_2} = \sqrt{\dfrac{ \sigma_1^2 }{n_1} + \dfrac{\sigma_2^2 }{n_2}} = \sqrt{\dfrac{ \sigma^2 }{n} + \dfrac{\sigma^2 }{n}} = \sqrt{\dfrac{ 2 \sigma^2 }{n}}