quick M1 question

For question 7iv, why is the value of T the smaller value of the quadratic? So, why is t 3 instead of 6??

http://pdf.ocr.org.uk/download/pp_11_jun/ocr_66474_pp_11_jun_gce_4728.pdf?
CONFUSED.

They only really collide once.

From the quadratic equation, it tells us they collide either at time 3 or 6.

If they collided when t=3, then they won't collide again.

So the time cannot be 6. Since they have already collided.

And the question says "P and Q collide first when t=T."

Hope this makes sense.