\[10ab - 5b^{2} - 6a + 3b =\]
\[= 5b \cdot (2a - b) - 3(2a - b) =\]
\[= (2a - b)(5b - 3)\text{\ \ }\]
\[при\ \ a = 6\frac{1}{5} = 6,2;\ \ \ b = 2,4:\]
\[(2 \cdot 6,2 - 2,4) \cdot (5 \cdot 2,4 - 3) =\]
\[= 10 \cdot 9 = 90.\]