Решение.
$$|\vec{a}| = \sqrt{(-3)^2 + 4^2} = \sqrt{9 + 16} = \sqrt{25} = 5$$
$$|\vec{b}| = \sqrt{5^2 + 0^2} = \sqrt{25 + 0} = \sqrt{25} = 5$$
$$|\vec{c}| = \sqrt{0^2 + (-2)^2} = \sqrt{0 + 4} = \sqrt{4} = 2$$
Ответ: |а| = 5; |b| = 5; |c| = 2.