\begin{align} t^2 &= (t^I)^2-\frac{v^2}{c^2}(t^I)^2 \\ t^2 &= (t^I)^2\left(1-\frac{v^2}{c^2}\right) \end{align}
I've been researching Einstein's Special Relativity Theory and would love to know the ins and outs of the equations. I haven't taken a algebra course in almost two decades and simplifying the equations was making sense until this came up (image related.) What mathematical principal is this called when rearranging the right side of the equation? It doesn't make sense at all. I don't need a direct explanation as much as what rule it is so I can further research it myself. Thanks for the help.