The modulus of $ z $ is:
$$ |z| = \frac{\sqrt{ 1061 }}{ 10 }$$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 = -1 $ and $ b = \frac{ 31 }{ 10 } $ so:
$$ \begin{aligned}|z| &= \sqrt{ (-1)^2 + \left(\frac{ 31 }{ 10 }\right)^2 } \\[1 em]|z| &= \sqrt{ 1 + \frac{ 961 }{ 100 } } \\[1 em]|z| &= \sqrt{ \frac{ 1061 }{ 100 } } \\[1 em]|z| &= \frac{\sqrt{ 1061 }}{ 10 } \\[1 em] \end{aligned} $$