X x $\frac{2}{3}$ + $\frac{3}{4}$ x X = $\frac{7}{8}$
X x ($\frac{2}{3}$ + $\frac{3}{4}$ ) = $\frac{7}{8}$
X x ($\frac{8}{12}$ + $\frac{9}{12}$ ) = $\frac{7}{8}$
X x $\frac{17}{12}$ = $\frac{7}{8}$
X = $\frac{7}{8}$ : $\frac{17}{12}$
X = $\frac{7}{8}$ x $\frac{12}{17}$
X = $\frac{21}{34}$