The length is 5.0. You could use the formula, or by realize that this is another 3-4-5 right triangle.

The formula works for vectors aligned with the axes:

length of((8,0)) = ( 8*8 + 0*0 ) = 8^{T}

It is awkward to keep saying "length of ( )." There is a symbol for this:

length of (a) =| a |

Your browser might not show this well.
The symbol **a** has a vertical bar on each side.
Sometimes books use two vertical bars on each side.
Using the new notation:

| (x, y)= ( x^{T}|^{2}+ y^{2})