given length BD is 15m and vector equation of line BD is

(-1) (15) i.e +lamda(15,-20,0)

(-7) + lamda (-20)

(11) (0)

find corordinates of D which is in mark scheme (8,-19,11)

This question is so easy but im so annoyed i cant understand it. I wouldnt have thought 15 comes from pythagarus of each point and someow thats incorporated, Obviously not as the mark scheme tells me to get lamda as 3 or 3/5 as appropriate(HOW) then sub in! where does lamda come from....????