The modulus of $ z $ is:
$$ |z| = \sqrt{ 65 }$$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 = -8 $ and $ b = 1 $ so:
$$ \begin{aligned}|z| &= \sqrt{ (-8)^2 + 1^2 } \\[1 em]|z| &= \sqrt{ 64 + 1 } \\[1 em]|z| &= \sqrt{ 65 } \\[1 em] \end{aligned} $$