[tex]7x^3a=\boxed{7}\cdot\boxed{x}\cdot x\cdot x\cdot\boxed{a}\\\\7xa^2=\boxed{7}\cdot\boxed{x}\cdot\boxed{a}\cdot a\\\\7x^3a+7xa^2=\boxed{7}\cdot\boxed{x}\cdot\boxed{a}\cdot x\cdot x+\boxed{7}\cdot\boxed{x}\cdot\boxed{a}\cdot a\\\\=\boxed{7}\cdot\boxed{x}\cdot\boxed{a}(x\cdot x+a)=7xa(x^2+a)[/tex]
