The Formula for The $f$ - Test for Comparing Two Means (Independent Samples, Variances Equal)

The formula for the $f$ - test for comparing two means (independent samples, variances equal)
$$t = \frac { \left( \bar { X } _ { 1 } - \bar { X } _ { 2 } \right) - \left( \mu _ { 1 } - \mu _ { 2 } \right) } { \sqrt { \frac { s _ { 1 } ^ { 2 } } { n _ { 1 } } + \frac { s _ { 2 } ^ { 2 } } { n _ { 2 } } } } .$$