Explanation:
there are many construction vids if you need a visual, but first off you need to a ruler, compass and protractor. draw a straight line horizontally, leave enough space above to add the length given as vertical side.
on the straight line choose a point preferably near centre and. place compass on the point and open to any distance, on the pencil side, draw a semi circle cutting the horizontal line at two points.
label the two points a and b
place compass on point and draw a smaller curve from that point above the center then do the same on other side
remember do not change the Size the compass was opened at for drawing the curves
the two new curves you drew should intersect, using ruler, draw a straight line vertically through intersection and center point
this line can be the required length given
you will notice an inverted T shape
the angle will be 90deg on either side, measure to ensure