\[\boxed{\text{28\ (28).}\text{\ }\text{Еуроки\ -\ ДЗ\ без\ мороки}}\]
\[\textbf{а)}\ a = 10;\ \ x = - 5;\ \ y = - \frac{1}{3}:\ \ \]
\[ax - 3y =\]
\[= 10 \cdot ( - 5) - 3 \cdot \left( - \frac{1}{3} \right) =\]
\[= - 50 + \frac{3 \cdot 1}{3} = - 50 + 1 =\]
\[= - 49.\]
\[\textbf{б)}\ a = \frac{1}{2};\ \ x = 2;\ b = - 3;\ \ \]
\[c = 5,8:\ \]
\[ax + bx + c =\]
\[= \frac{1}{2} \cdot 2 - 3 \cdot 2 + 5,8 = \ \]
\[= 1 - 6 + 5,8 = 6,8 - 6 = 0,8.\]