To do this you can use the following formula that is used to standardize a random variable which has a normal distribution
[tex]Z=\frac{x-\mu}{\sigma}[/tex]Where
[tex]\begin{gathered} \mu\text{ represents the mean and, } \\ \sigma\text{ represents the standard desviation} \end{gathered}[/tex]So, replacing you have
[tex]Z=\frac{x-\mu}{\sigma}=\frac{50-58}{12}=-\frac{8}{12}[/tex]Simplifying you have
[tex]Z=-\frac{8}{12}=-\frac{2}{3}=-0.67[/tex]