The modulus of $ z $ is:
$$ |z| = 80 \sqrt{ 10 }$$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 = 240 $ and $ b = -80 $ so:
$$ \begin{aligned}|z| &= \sqrt{ 240^2 + (-80)^2 } \\[1 em]|z| &= \sqrt{ 57600 + 6400 } \\[1 em]|z| &= \sqrt{ 64000 } \\[1 em]|z| &= 80 \sqrt{ 10 } \\[1 em] \end{aligned} $$