`u = -21i` Use the dot product to find the magnitude of u.

The magnitude of a vector `u` is the square root of its dot product by itself, because

`u*u=||u||*||u||*cos(theta),` and `theta=0,` `cos(theta)=1.`

I suppose that the magnitude of `i` is 1. Then

`||u||=sqrt(u*u)=sqrt((-21i)*(-21i))=sqrt(21*21)=21.`

This is the answer.

The magnitude of a vector `u` is the square root of its dot product by itself, because

`u*u=||u||*||u||*cos(theta),` and `theta=0,` `cos(theta)=1.`

I suppose that the magnitude of `i` is 1. Then

`||u||=sqrt(u*u)=sqrt((-21i)*(-21i))=sqrt(21*21)=21.`

This is the answer.