This vector question has three parts, and I've done the first two, finding the point of intersection and acute angle between them. It's the third bit I don't understand.

L1 = 2i-j+3k + s(j-k)

L2 = -2i+7j+3k + t(2i-3j-k)

iii) The foot of the perpendicular from the point with position vector -2i+7j+3k to the line L1 is P. Find the position vector of P.

I just don't really understand what this means, any help would be much appreciated.