The area of a triangle is proportional to its base, so if x is the area of triange ACD, then (x + 20 + 30)/9 = x/7 ==> x = 175.
+118608 That is a good observation guest. 
Let CD be x units in length and let the height be h
so what is BC?
what is DE?
You need to find AreaACD = xh/2
Set up formulas for the two areas that you know.
Solve them simultaneously to find the value of xh
Note: you probably can solve for x and h independently but it is easier to find the value of xh
