The modulus of $ z $ is:
$$ |z| = 4 \sqrt{ 3 }$$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 = -6 $ and $ b = 2 \sqrt{ 3 } $ so:
$$ \begin{aligned}|z| &= \sqrt{ (-6)^2 + \left(2 \sqrt{ 3 }\right)^2 } \\[1 em]|z| &= \sqrt{ 36 + 12 } \\[1 em]|z| &= \sqrt{ 48 } \\[1 em]|z| &= 4 \sqrt{ 3 } \\[1 em] \end{aligned} $$