I believe the answer should the 10.
Looking at the two given triangles, the singular line in the bottom left triangles means that the angles are the same. As to the corner seen in cornerF and cornerC, and cornerD with cornerA. Knowing that all of the angles in the triangle are the same, means that the side lengths should all be generally on the same scale.
With triangle DEF, this triangle got scaled up by 2. This can be seen from line AB and line DE. AB was multiplied by two, in order to get line DE, which is 8. Apply the x2 rule to lines DF and EF, and the unknown "e" turns into 10. Unkown "d" turns into 12.
