The Brainliest Answer!
First u draw a number line

then draw a line between 0 and 2( 2 unit)
then draw 3 perpendicular line from the point 2
then take 3 cm on compuss and draw a   arch    to the perpendicular line
joint this line
u will get a right-angled triangle
so the perpendicular line=√5
again take ur compass and draw it on ur number will be √5
see attached file

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AC looks like 3 units...
your answer is right...
but that is not the standard procedure taught in cbse....
you should take AC as 1 unit and then AB will be root 5 unit..that is easy to understand too
1] Draw number line and take a point O representing O on it. 
Take a point A on the number line such that OA = 1(units) 

2] Take a point B on the number line such that AB = 2(units) 

3] With centre B draw a right angled triangle ABC(right angled at A) such that AB =2 and BC =1 

4] With A as centre and radius AC draw an arch that cuts the number line at P 

5] Then, AB = AP = √5 
6] OC =OA+AP=1+√5.

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