a)
[tex]\dfrac{9^{11}\cdot2+4\cdot9^{11}}{(9^7:81)\cdot3^2}=\dfrac{9^{11}\cdot(2+4)}{(9^7:9^2)\cdot9}=\dfrac{9^{11}\cdot6}{9^{7+2}\cdot9^1}=\dfrac{9^{10+1}\cdot6}{9^{9+1}}=\\\\{}\qqud\qquad\qquad\qquad\qquad\qquad\qquad\qquad \qquad\qquad =\dfrac{9^{10}\cdot9^1\cdot6}{9^{10}}=\dfrac{1\cdot9\cdot6}{1}=54[/tex]
b)
[tex]\big3^\frac13\cdot\big9^\frac13-\big5^\frac14\cdot12\big5^\frac14= \big3^\frac13\cdot\left(3^2\right)^\frac13-\big5^\frac14\cdot\left(5^3\right)^\frac14 = \big3^\frac13\cdot\big3^{2\cdot\frac13}-\big5^\frac14\cdot\big5^{3\cdot\frac14}= \\\\ =\big3^\frac13\cdot\big3^\frac23-\big5^\frac14\cdot\big5^\frac34= \big3^{\frac13+\frac23}-\big5^{\frac14+\frac34}=\big3^1-\big5^1=3-5=-2[/tex]
