How do you make under root 17 on number line?

Go to the number 4 on the number line. Draw a vertical line at this point: going up at 90 degrees to the number line. Measure 1 unit [ = the distance from 0 to 1] on this line and mark it as Y.

Take a compass and put the point at 0. Take the pencil to the point Y and draw an arc to intersect the number lie at the point X. Then X is the required point, at sqrt(17).