Can anyone me on how to as we this

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.