$a)$  

$y=\frac{3}{5}x-6$  

$\mathrm{tg}\alpha =\frac{3}{5}=0,6$  

$\alpha \approx {31}^{\circ }$  

 

$b)$  

$y-4x+8=0$  

$y=4x-8$  

$\mathrm{tg}\alpha =4$  

$\alpha \approx {76}^{\circ }$  

 

$c)$  

$2y+6x+\frac{1}{4}=0$  

$2y=-6x-\frac{1}{4}$  

$y=-3x-\frac{1}{8}$  

$\mathrm{tg}\alpha =-3$  

$\mathrm{tg}\left({180}^{\circ }-\alpha \right)=-\mathrm{tg}\alpha =3$  

${180}^{\circ }-\alpha \approx {72}^{\circ }$  

$\alpha \approx {180}^{\circ }-{72}^{\circ }={108}^{\circ }$