Help with GCSE vectors question

Hello, there's a vector question I'm stuck on.

D, E and F are 3 points on a straight line such that

Vector DE = 3e + 6f
Vector EF = -10.5e - 21f

Find the ratio
Length of DF: length of DE

I get that if DE and EF are on the straight line, that DF = (3e + 6f) + (-10.5e - 21f), which is -7.5f -15f.

So -7.5f -15f : 3e + 6f

But I don't understand what to do next, can anyone help?

What do you notice about the ratios for each of the e and f coefficients?
It may help to just sketch a simple, exemplar line to get the right argument.

You have to times EF by -2.5 to get DE? So is the answer -2.5:1?