$\mathrm{tg}\alpha =0,75=\frac{3}{4}$  

$\frac{\sin \alpha }{\cos \alpha }=\frac{3}{4}\mid \cdot \left(\cos \alpha \right)$  

$\sin \alpha =\frac{3}{4}\cos \alpha$  

 

${\sin }^2\alpha +{\cos }^2\alpha =1$  

${\left(\frac{3}{4}\cos \alpha \right)}^2+{\cos }^2\alpha =1$  

$\frac{9}{16}{\cos }^2\alpha +{\cos }^2\alpha =1$  

$\frac{25}{16}{\cos }^2\alpha =1\mid \cdot \frac{16}{25}$  

${\cos }^2\alpha =\frac{16}{25}$  

$\cos \alpha =\sqrt{\frac{16}{25}}=\frac{4}{5}$  

 

$\sin \alpha =\frac{3}{4}\cdot \frac{4}{5}=\frac{3}{5}$  

 

$2\cos \alpha -10\sin \alpha =2\cdot \frac{4}{5}-10\cdot \frac{3}{5}=\frac{8}{5}-\frac{30}{5}=-\frac{22}{5}=-4,4$