[tex]\displaystyle\bf\\3ab^{2} \cdot(a-2b+ab)=3ab^{2} \cdot a-3ab^{2} \cdot 2b+3ab^{2} \cdot ab=\\\\\\=3a^{2} b^{2} -6ab^{3} +3a^{2} b^{3}[/tex]