[tex]MN = \sqrt{ (x_{N} - x_{M} ) {}^{2} + (y_{N} - y_{M} ) {}^{2} } [/tex]
[tex]MN = \sqrt{ (3 - ( - 3) ) {}^{2} + (4 - ( - 4) ) {}^{2} } [/tex]
[tex]MN = \sqrt{( 3 + 3) {}^{2} + (4 + 4 ) {}^{2} } [/tex]
[tex]MN = \sqrt{ 6 {}^{2} + 8 {}^{2} } = \sqrt{100} = 10cm[/tex]