Okay, I am telling the answer using pythagoras theorem
take the two perpendicular sides of the first triangle as 1 and 1 units
now you can find hypotenuse = root 2 units.
Now measure root 2 [hypotenuse of the above traingle] and mark on another sheet or the same sheet if you want. Now draw 1 unit perpendicular to it and find its hypotenuse too, that will be root 3 units.
Now measure root 3 units from the hypotenuse of above triangle using compass and mark it somewhere else. Now make a perpendicular of 2 units to root 3 units and now find its hypotenuse too using the pythagoras theorem and you will get root 7, now go and mark root 7 on the number line ! :-)
[See the photo below, I made it to make it clear]
Good if understood, if not, please comment below :-)