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