Ahsan, please don't mind i think your answer is probably wrong.
By using acd's idea i have solved the question, which is as follows.
Area of the rectangle above the line y=x
P(x
Total area of the rectangle
2.5
= ----------
2*3
5/2
= ----------
6
5
= -------
12
A better solution with figure is in attached file. I think it is right answer.
Thanks acd, for the idea. solution.doc