The modulus of $ z $ is:
$$ |z| = 15$$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 = -15 $ so:
$$ \begin{aligned}|z| &= \sqrt{ 0^2 + (-15)^2 } \\[1 em]|z| &= \sqrt{ 0 + 225 } \\[1 em]|z| &= \sqrt{ 225 } \\[1 em]|z| &= 15 \\[1 em] \end{aligned} $$