Ahh this is a good hard question, I hope someone comes along and helps. Goodluck my friend!
Es que es muy difícil saber eso hola soy janellys1230 ahora
So 39 books and it must equal 70. So 39 + ___ = 70 or, to make it a bit simpler, 70 - 39 = ___. You should be able to do that.
Here is your answer

REASON:
Theorem used: The diagonals of a parallelogram bisect each other.
Let diagonals AC and BD bisect each other at O
So, OA=OC
Now,
3x=4x-6 [OA=3x and OC=4x-6]
4x-3x= 6
x= 6
HOPE IT IS USEFUL