The modulus of $ z $ is:
$$ |z| = \frac{\sqrt{ 791 }}{ 7 }$$To find modulus of a complex number $ z = a + bi $ we use formula:
$$ |z| = \sqrt{a^2 + b^2} $$In this example we have $ a = \frac{\sqrt{ 7 }}{ 7 } $ and $ b = -4 $ so:
$$ \begin{aligned}|z| &= \sqrt{ \left( \frac{\sqrt{ 7 }}{ 7 } \right)^2 + (-4)^2 } \\[1 em]|z| &= \sqrt{ \frac{ 1 }{ 7 } + 16 } \\[1 em]|z| &= \sqrt{ \frac{ 113 }{ 7 } } \\[1 em]|z| &= \frac{\sqrt{ 791 }}{ 7 } \\[1 em] \end{aligned} $$