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)T ) = ( 8*8 + 0*0 ) = 8
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)T | = ( x2 + y2 )