What is the modulus of the complex number z=3+3i?

$\left\mid 3 + 3 i \right\mid = \sqrt{{3}^{2} + {3}^{2}} = \sqrt{{3}^{2} \cdot 2} = 3 \sqrt{2}$
In general $\left\mid a + i b \right\mid = \sqrt{{a}^{2} + {b}^{2}}$