The modulus of $ z $ is:
$$ |z| = 10$$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 = -6 $ and $ b = 8 $ so:
$$ \begin{aligned}|z| &= \sqrt{ (-6)^2 + 8^2 } \\[1 em]|z| &= \sqrt{ 36 + 64 } \\[1 em]|z| &= \sqrt{ 100 } \\[1 em]|z| &= 10 \\[1 em] \end{aligned} $$