Answer: [tex]M=\frac{gu^2}{2}-N[/tex]
Work Shown:
[tex]\frac{u}{\sqrt{2}} = \left(\frac{M+N}{g}\right)^{0.5}\\\\\\\left(\frac{M+N}{g}\right)^{0.5}=\frac{u}{\sqrt{2}}\\\\\\\sqrt{\frac{M+N}{g}}=\frac{u}{\sqrt{2}}\\\\\\\frac{M+N}{g}=\left(\frac{u}{\sqrt{2}}\right)^{2}\\\\\\\frac{M+N}{g}=\frac{u^2}{(\sqrt{2})^2}\\\\\\\frac{M+N}{g}=\frac{u^2}{2}\\\\\\M+N=\frac{gu^2}{2}\\\\\\M=\frac{gu^2}{2}-N\\\\\\[/tex]
