Let $\mu$ and $\sigma$ be the mean and the standard deviation of the distribution.
$\left\{ \begin{array}{ll}
\dfrac{46-\mu}{\sigma} = -3 & \ldots \unicode{x2460} \\
\dfrac{x-\mu}{\sigma}=1 & \ldots \unicode{x2461} \\
\dfrac{86-\mu}{\sigma}=2 & \ldots \unicode{x2462}
\end{array}\right.$
$\unicode{x2462} \div \unicode{x2460}$, we have
$\begin{array}{rcl}
\dfrac{86-\mu}{46-\mu} & = & \dfrac{2}{-3} \\
-3(86-\mu) & = & 2(46-\mu) \\
-258+3\mu & = & 92 -2\mu \\
5\mu & = & 350 \\
\mu & = & 70
\end{array}$
Sub. $\mu =70$ into $\unicode{x2462}$, we have
$\begin{array}{rcl}
\dfrac{86-70}{\sigma} & = & 2 \\
\sigma & = & 8
\end{array}$
Sub. $\mu=70$ and $\sigma=8$ into $\unicode{x2461}$, we have
$\begin{array}{rcl}
\dfrac{x-70}{8} & = & 1 \\
x-70 & = & 8 \\
x & = & 78
\end{array}$
