Knowing that GB is 18 and CD = 21 you should be able to use them to solve this problem.

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Step-by-step explanation:

Point G is the intersection of medians, the centroid. That point divides each median into parts with a 2:1 ratio. The "2" part is the distance between the centroid and the vertex. The "1" part is the distance between the centroid and the side.

  FG : GB : BF = 1 : 2 : 3 = 9 : 18 : 27 . . . . . where 18 is given

  GD : CG : CD = 1 : 2 : 3 = 7 : 14 : 21 . . . . . where 21 is given