Question

how to denote root under 5 on number line

Answer 1

You can use Pythagoras theorem as below
$\sqrt{5} = \sqrt{ {1}^{2} + { 2 }^{2} }$
So draw a perpendicular line of length 1 unit from point (2,0) in the number.
Join the tip of the perpendicular line and origin.
Now you have right angled triangle whose hypotenus is equal to square root of 5.
To get this value on the number line, use a compass.