The figure below shows a triangular wooden frame abc. the side ad of the frame has rotted and needs to be replaced what is the length of the wood that is needed to replace ad?

So hmm notice the picture below

the 15° angle  length or AD, will be the difference of sides AB - DB

since subtracting the greater angle from the smaller, will simply give you their difference, or the 15° angle

so, using the  45-45-90 and 30-60-90 ratios, we get the value of "x"

and we know AB = 12, so, DB  = x

now  

4should be the last one n=4