In the diagram, BD = 5, CE = 4, [ABC] = 24, and [ADE] = 18. Find [ACD].
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.
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