Calling all you mathematicians

Challenge 3- Maths.(look at the sidebar)
Yesterday I sat down with a question. But it was damn difficult. It made me go insane ( "crazy" would be a better adj.)
But at last i found the answer.


Though its not to difficult, yet i Have rated it 3stars.
Just one suggestion, when you sit with this question, make sure there is total silence.Not even 1db sound.:)
Lol