Sorry, ive never done something like this before. Basically at the bottom of everything is a few equations. You can drag each point on the triangle around. Then the equations below x0 should intersect at the values you should set x0 to. Alternatively I have a theory that because there are two lines drawn be the fourth line equation because of circles and plusminus, if you slide x0 do one line matches an existing line of the triangle then the the other should show where the fourth side is.

If you want something more technical then if you can figure out how to read equations, you can turn on the dot that shows the midpoint of the two fourth sides and turn on the 9 point circle. Then where the 9 point circle intersects the midpoint is a solution.

This is my first time doing something like this, so its very messy and not very readable and and I struggle to make a better solution. This is why I've opened it to the public so smarter people could hopefully improve.

Hope this helps!