Trig expression problem

[stuck bear]

Not sure how to do this, what does it mean?

Ive tried experimenting with tangent expressions and what not but i just cant figure it out for the life of me

Just really an egregious piece of a problem, im sure theres likely a simple route to solving it though.

[shut pulsar]

let's call the height below a is y, then:
tan(D) = y/b
y = b tan(D)
tan(C+D) = (a+y)/b
a+y = b tan(C+D)
a = b tan(C+D) - y
a = b tan(C+D) - b tan(D)