It is given that

$$ z=\frac{x-\mu}{\sigma} $$

Also,

$$ \mu=78,\quad \sigma=6,\quad z=2 $$

So by plugging in the values, we get

$$ 2=\frac{x-78}{6} $$

$$ 12=x-78 $$

$$ x=12+78 $$

$$ x=90 $$