Answer
$x$ = $58.96$
Work Step by Step
i ) z score corresponding to the 90th percentile (area of 0.90) = 1.28
ii) Sub $\sigma = 7 , \mu= 50, z = 1.28$ into z = $\frac{x - \mu}{\sigma}$ and solve for x:
z = $\frac{x - \mu}{\sigma}$
$x$ = $\mu$ + $z\sigma$
$x$ = $50 + (1.28 \times 7)$
$x$ = $58.96$