Step-by-step answer:
The first step in solving a problem of geometry and trigonometry is to draw a diagram, with all the given information in it. The attached image does just that.
Here, we have (horizontal) distance of building to the tower is 400'.
We see two right triangles, each with a leg of 400' (horizontal).
The vertical distances are denoted h1 and h2 for the angles of elevation 24 and 20 degrees respectively.
Using the definition of tangent, we have
h1= 400tan24=178.1
h2=400tan20=145.6
The height of the tower is therefore h1+h2=178.1+145.6=323.7 feet