[tex]T=2\pi\sqrt{\dfrac{m}{k}}\ \ \ \ \ |divide\ both\ sides\ by\ 2\pi\\\\\sqrt{\dfrac{m}{k}}=\dfrac{T}{2\pi}\ \ \ \ \ \ |square\ both\ sides\\\\\dfrac{m}{k}=\dfrac{T^2}{4\pi^2}\ \ \ \ \ \ |multiply\ both\ sides\ by\ k\neq0\\\\\boxed{m=\dfrac{T^2k}{4\pi^2}}[/tex]
