I'm assuming the loading dock is the same height as the rightmost vertical support. In that case, we can use similar triangles.
Let x = the height of the rightmost vertical support.
Then the following proportion must be true:
(1.2)/3 = x/5
(It is 5 because that is the base of the larger right triangle).
Here you can use cross multiplaction to solve for x.
x = (1.2)*5*1/3 = 2
The height is 2 meters tall.