Let the vertical line of F and E be CD, and the intersection point be G and H respectively.

Thus, fg/CG = 1/0.8, eh/HD =11.2.

Obviously, FG=EH, and it is the expected value.

CG+HD=FG*0.8+EH* 1.2=2*FG

CG+HD = CD-EF = 6-3.8 = 2.2。

So 2*FG=2.2

FG= 1. 1

That is, dam heightening1.1m.