When two or more points are called collinear, it means they are all lying on the same straight line.
Therefore, the question simply means all three points D, E and F all lie on the same straight line.
If the point E is somewhere between D and F, then DE + FE = DF
Therefore
DE + 11.9 = 17.6
Subtract 11.9 from both sides of the equation
DE = 5.7
