Hi Dasith, sorry for the late reply... >< I was a little tied up with my exams...
I shall include my working for the second part of the question here, the one on finding OX:
OX=OP+PX
=p + 1/3(-p+q)
=p - 1/3p+ 1/3q
=2/3p + 1/3q
When I saw that Ricky said that the answer is '1/3(2q+p)' , I was stumped as it was different from the answer I got above. However, I checked the answer scheme, which said that my answer is correct. It's actually '1/3(2p+q)... Do you see any difference between your working and mine?
Do let me know if you have any further difficulties...