i don't really know how to write this,
so here is a picture of the problem.
can u please help me ?
It's not really the policy of MHF to provide full solutions to problems.
Skeeter has given a pretty big hint - using what has been given the solution falls into place.
If your friend is still having trouble, please ask him/her to log on him/herself, show his/her working and exactly what point he/she gets stuck on.