Physics paper 6 i need help? anyone?

[Osmix1995]

so i was solving this paper
www.xtremepapers.com/papers/CIE/Cambridge%20IGCSE/Physics%20(0625)/0625_s10_qp_61.pdf
(may/june 2010 paper 61)
i came to question 4 i solved part a) easily
and then came part b) i don't understand it
can someone solve part b and post it or explain it for me??

 

[imperial]

use the doubts thread that is in the CIE sub-forum, ppl answered this, just search