If you  take  random point and across would be like this
             0( imagine "0" as a dot/point  as it more visible
this would be the across of the point 
and the angle of depression is a downward slope and it says 30 degrees
 so join those two points and mark the internal angle as 30 degrees now the first point as 4 degrees .
now there are saying that the bridge is of a height 4m so draw a straight line from point B(second point).now join all the points.(note: it forms a triangle im unable to draw over here think carefully and observe properly).if u dont know this property now you would be learning one.the sum of interior angles of a triangle is 180 degrees.
so it 180-30-45=105 degrees.and the width is = area of a triangle =1/2 multiplied by base  multiplied by width(bredth) 
180= 1/2 multiplied by 3 mutiplied bywidth(note: in this context 180 would be the area as the length and other figures are unknown).
so its 360 divided by 3
=120m which is the width
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