[tex] \frac{7}{x} + \frac{9}{12} = \frac{x}{12}[/tex]
[tex] \frac{7*12}{12*x}+ \frac{9*x}{12*x} = \frac{x}{12} [/tex]
[tex] \frac{84+9x}{12x} = \frac{x}{12} [/tex]
[tex] \frac{84+9}{12} = \frac{x}{12} [/tex]
[tex] \frac{93}{12} = \frac{x}{12} [/tex]
==> x = 93
