A hot-air balloon is floating above a straight road. To calculate their height above the ground, the balloonists simultaneously measure the angle of depression to two consecutive mileposts on the road on the same side of the balloon. The angles of depression are found to be 21 degrees and 23 degrees.

How high (in feet) is the ballon?

I have no clue how to solve for this without knowing any sides. Also need to know the formula to use and how to use it.