The 4 is the vertical beam, and the 3 is the horizontal beam. The horizontal beam is divided in two. Let us find where the division of 4 equals the division of 3 so that we know where to put the horizontal beam on the vertical! Let us find "t"!

The horizontal beam is 3, but seen together with 4 the beam is 3/4. H in the original is 4 - the height.

t = (3/4)H /2

t = 3/8 H