You need a compass.
1. Draw the first line to the length you want.
2. Stick the compass in the end point and adjust it to the length of the line. Draw a circle.
3. Stick the compass in the other end point and draw a secong circle.
4. Where the circles intersect is the point of your triangle. Connect the end points and you got it.
OR
1. Draw the first line to the length you want it and mark the center point.
2. Draw a perpendicular line from the center point. (90 degrees up or down)
3. Measure the length of the first line and find the point on the perpendicular line that gives you the same length to one of the end points. Sorry - a bit convoluted to explain. I use a ruler and rotate it on the end point until I "get there"
Hope this helps.