[ \dfrac{1}{7!} + \dfrac{1}{8!} = \dfrac{x}{9!} ] [ \dfrac{8\cdot9 + 9}{9!} = \dfrac{x}{9!} \implies 72 + 9 = x \implies x = 81. ]
(More carefully: ( \dfrac{1}{7!} = \dfrac{8\cdot9}{9!} ), ( \dfrac{1}{8!} = \dfrac{9}{9!} ), sum ( \dfrac{72+9}{9!} = \dfrac{81}{9!} ).)
Answer: ( x = 81 )