\(a)\) \(3x\left( {{x^2} - 7x + 9} \right)\)\(=3x.x^2-3x.7x+3x.9\) \( = 3{x^3} - 21{x^2} + 27x\)
\(b)\) \(\displaystyle{2 \over 5}xy\left( {{x^2}y - 5x + 10y} \right)\)\(=\displaystyle {2 \over 5}xy.xy^2-{2 \over 5}xy.5x+{2 \over 5}xy.10y\) \( = \displaystyle{2 \over 5}{x^3}{y^2} - 2{x^2}y + 4x{y^2}\)
