Answer:
ac= 21
Step-by-step explanation:
Rhombus is a parallelogram in which diagonals bisect each other .
Given: Area of rhombus is 168 .
The diagonals of rhombus abcd intersect at point e. It's diagonals are ac and bd . We know that diagonals of rhombus bisect each other ,so, bd=2de . So,
.
Also, area of rhombus is
where
are the diagonals of rhombus .
As area of rhombus is 168 , we get ,

Let 
So, we get ,

Therefore, 