\(a,A=\dfrac{\left(3-\sqrt{x}\right)\left(\sqrt{x}-2\right)}{\left(\sqrt{x}+2\right)\left(\sqrt{x}-2\right)}=\dfrac{5\sqrt{x}-6-x}{x-4}\\ b,x=\dfrac{4}{9}\Leftrightarrow\sqrt{x}=\dfrac{2}{3}\Leftrightarrow A=\dfrac{5\cdot\dfrac{2}{3}-6-\dfrac{4}{9}}{\dfrac{4}{9}-4}\\ A=\dfrac{\dfrac{10}{3}-\dfrac{4}{9}-6}{-\dfrac{32}{9}}=\dfrac{-28}{9}:\dfrac{-32}{9}=\dfrac{7}{8}\)


