$m=\sqrt{ 42 }$
Write the solutions, one with a $+$ sign and one with a $-$ sign$\begin{array} { l }m=-\sqrt{ 42 },\\m=\sqrt{ 42 }\end{array}$
The equation has $2$ solutions$\begin{align*}&\begin{array} { l }m_1=-\sqrt{ 42 },& m_2=\sqrt{ 42 }\end{array} \\&\begin{array} { l }m_1\approx-6.48074,& m_2\approx6.48074\end{array}\end{align*}$