The magnitude of a vector is given by the square root of the sum of the squares of its components. So, for the vector $\mathbf{u}=\langle-1,5,2\rangle$, the magnitude is given by:
$$
||\mathbf{u}|| = \sqrt{(-1)^2 + 5^2 + 2^2} = \sqrt{30}
$$
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