According to the Pythagoras theorem, we can conclude that

( √5 )^{2} = ( 2 )^{2} + ( 1 )^{2}.

We need to draw a line segment AB of 1 unit on the number line. Then draw a straight line segment BC of 2 units. Then join the points C and A, to form a line segment BC.

Then draw the arc ACD, to get the number √5 on the number line.