To find the length of line cf, I first created a square and labeled the vertices a b c d. From here you know the triangle shares the vertice b. I used the lines on the square to add point e. The distance between a and e was 2, leaving us with a distance of 4 from a to b.. I used the 4 to find the missing height. knowing that the total area was 20, the height had to be 10. I extended side cb of the triangle and made a point f. The extra part measured 4 units because the side of the square was 6, so to get 10 I needed 4 more units. I have attached a picture showing this.