a: \(A=\left(28\sqrt{3}+9\sqrt{3}-4\sqrt{3}\right)\cdot\sqrt{3}\)
\(=33\sqrt{3}\cdot\sqrt{3}=33\cdot3=99\)
b: \(B=12\sqrt{5}-8\sqrt{20}+7\sqrt{45}\)
\(=12\sqrt{5}-16\sqrt{5}+21\sqrt{5}=17\sqrt{5}\)
c: \(C=36-12\sqrt{18}+15\sqrt{12}-\dfrac{3}{4}\cdot\sqrt{48}\)
\(=36-36\sqrt{2}+30\sqrt{3}-\dfrac{3}{4}\cdot4\sqrt{3}\)
\(=36-36\sqrt{2}+27\sqrt{3}\)