a: \(A=7\cdot12+\sqrt{\dfrac{257}{64}}=84+\dfrac{\sqrt{257}}{8}\)
b: \(B=\dfrac{\left(2\sqrt{3}+5\sqrt{3}+3\sqrt{3}\right)}{\sqrt{15}}=\dfrac{10\sqrt{3}}{\sqrt{15}}=\sqrt{20}=2\sqrt{5}\)
c: \(C=\left(\dfrac{1}{\sqrt{7}}-\dfrac{4}{\sqrt{7}}+\dfrac{3}{\sqrt{7}}\right):\sqrt{7}=0\)