[tex]cos\alpha=-\frac34\\-\frac34=\frac{m}r\\\\sin^2\alpha+cos^2\alpha=1\\sin^2\alpha+(-\frac34)^2=1\\sin^2\alpha=1-\frac9{16}\\sin^2\alpha=\frac{7}{16}\\sin\alpha=\frac{\sqrt7}4\\\\\frac{\sqrt7}4=\frac{7}r\\\\r=\frac{7*4}{\sqrt7}=\frac{28}{\sqrt7}=\frac{28\sqrt7}7=4\sqrt7\\\\-\frac34=\frac{m}{4\sqrt7}\\\\m=\frac{-3*4\sqrt7}4\\m=\frac{-12\sqrt7}4\\m=-3\sqrt7[/tex]
