Trionometry with a bit of pythag

Hi i am new here.

Can anyone point me in the right direction on this one please

http://i75.photobucket.com/albums/i3...scan0002-1.jpg

b=15.7

I have gone as far as splitting the 2 triangles up and labeling the 2

24.3² + DB = BC²

15.7² + DB² = AB² (DB being the vertical unknown side)

I then did AB + BC = 42 so thefore AB=42-BC and BC=42-AB

I substituted these values in to the equation i came up with

24.3² + DB = (42-AB)

15.7² + DB² = (42-BC)

I then tryed subtracting these away from each other as if a symlytaineous equation. (dont know if that was the right way to go about it)

Thanks