The modulus of $ z $ is:
$$ |z| = 21 \sqrt{ 2 }$$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 = 7 \sqrt{ 18 } $ and $ b = 0 $ so:
$$ \begin{aligned}|z| &= \sqrt{ \left(7 \sqrt{ 18 }\right)^2 + 0^2 } \\[1 em]|z| &= \sqrt{ 882 + 0 } \\[1 em]|z| &= \sqrt{ 882 } \\[1 em]|z| &= 21 \sqrt{ 2 } \\[1 em] \end{aligned} $$