[tex]\displaystyle\bf\\-12a^{2} b\Big(-\frac{1}{3} ab^{3}\Big)^{2} =-12a^{2} b\cdot\frac{1}{9} a^{2} b^{6}=-12\cdot\frac{1}{9} \cdot a^{2+2} \cdot b^{1+6} =\\\\\\ =-1\frac{1}{3} a^{4} b ^{7}[/tex]