The modulus of $ z $ is:
$$ |z| = 6$$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 = 3 \sqrt{ 3 } $ and $ b = -3 $ so:
$$ \begin{aligned}|z| &= \sqrt{ \left(3 \sqrt{ 3 }\right)^2 + (-3)^2 } \\[1 em]|z| &= \sqrt{ 27 + 9 } \\[1 em]|z| &= \sqrt{ 36 } \\[1 em]|z| &= 6 \\[1 em] \end{aligned} $$