The modulus of $ z $ is:
$$ |z| = 5$$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 $ and $ b = -4 $ so:
$$ \begin{aligned}|z| &= \sqrt{ 3^2 + (-4)^2 } \\[1 em]|z| &= \sqrt{ 9 + 16 } \\[1 em]|z| &= \sqrt{ 25 } \\[1 em]|z| &= 5 \\[1 em] \end{aligned} $$