The key to answering the question is to notice that the entire figure is a parallelogram and that EF is the parallel mid segment that divides the parallelogram into 2 symmetric/congruent areas.
Thus, we have parallelogram ABEF = Area of (1/2) (36) = 18
Rule: between 2 parallel lines, the perpendicular height will always be the same.
Call the area of triangle AEX = [AEX]
Area of triangle ABX = [ABX]
Area of triangle BFX = [BFX]
[AEX] + [ABX] + [BFX] = 18 (equation 1)
Choose base sides for each triangle:
EX = 6
Let XF = b
Since the opposite sides of a parallelogram are equal, AB = 6 + b
And all 3 triangles will have an equivalent perpendicular height drawn to the respective bases above ——> call it H = perpendicular distance between the 2 parallel lines AB and EF
[AEX] = (1/2) (6) (H) = 6H / 2
[BFX] = (1/2) (b) (H) = bH / 2
[ABX] = (1/2) (b + 6) (H) = (6H + bH) / 2
As you can see, the Area of triangle AEX + the Area of triangle BFX = the target triangle’s Area, triangle ABX
[AEX] + [BFX] = [ABX] (equation 2)
Combining this with (equation 1) above
[AEX] + [BFX] + [ABX] = 18
Substitute (equation 2) into (equation 1)
[ABX] + [ABX] = 18
(2) * [ABX] = 18
Area of triangle ABX = [ABX] = 9
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