`u = 6j` 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((6i)*(6i))=sqrt(6*6)=6.`

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((6i)*(6i))=sqrt(6*6)=6.`

This is the answer.