Welcome to my contest #4 ! Take your share of 50% of upvotes

This is my fourth contest, i need to know what is value in the equation.

To enter this contest you have to upvote the post and comment with the right answer.
Please keep it simple and do not explain how you found the right answer, if you do you will NOT get the prize !
After 7 days i will distribute the prize to the winners.
The prize will be 50% of the sbd made with the post.
Only one entry per person is allowed!
Cheers and Good luck everyone :)