The modulus of $ z $ is:
$$ |z| = 4$$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 = \sqrt{ 7 } $ and $ b = -3 $ so:
$$ \begin{aligned}|z| &= \sqrt{ \left( \sqrt{ 7 } \right)^{ 2 } + (-3)^2 } \\[1 em]|z| &= \sqrt{ 7 + 9 } \\[1 em]|z| &= \sqrt{ 16 } \\[1 em]|z| &= 4 \\[1 em] \end{aligned} $$