Look at the angles and the little tallies to match up the triangle. Or use the given: triangle DEF is similar to triangle KJI. If it’s like that in a given you know the order is equal. (D and K are similar, E and J, and F and I)
So if line FD is the 3rd and 1st letters of the first triangle, then it has to be equal to the 3rd and 1st letter of the 2nd triangle.
IK
Another way to do this is that when you find FD on the triangle you can set that the line has 3 tally marks. Then look at the other triangle and find a line in which there are 3 tally marks.
The order in which you put the letters does not matter. IK and KI should both be considered correct
Hope this helped