Transitivity of parallel lines

Transitivity of parallel lines

I cam across a question (in my textbook) about proofs with parallel lines.
The question is: Prove that the property that || is transitive implies
that for any point P and line l, there is at the most 1 line through P
that is parallel to the line l.
In other words the question is asking me to prove that (P is on q & P is
on s & l||q & l||s) --> q = s, using the transitive property.
I know that the transitivity property tells me that l||m & m||n --> n||l,
but I am not sure how to do this proof.
Any hints or solutions would be much appreciated!! :)
Thank you in advance