The modulus of $ z $ is:
$$ |z| = 7$$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 = 0 $ and $ b = -7 $ so:
$$ \begin{aligned}|z| &= \sqrt{ 0^2 + (-7)^2 } \\[1 em]|z| &= \sqrt{ 0 + 49 } \\[1 em]|z| &= \sqrt{ 49 } \\[1 em]|z| &= 7 \\[1 em] \end{aligned} $$