Let \mathbf a = (-2, -7, -1)

Question

Let a = (-2, -7, -1)

 

|a|=

Answer 1

I confirm that Victoria's answer is correct.  Clearly, someone abused the voting system by casting an unjustified down-vote against her answer.

Answer 2

The magnitude of a 3-dimensional vector is sqrt(x²+y²+z²)

 

So your magnitude will be sqrt(4+49+1) = sqrt(54) = 3√6