Question

how to find perimeter of rectangle with area?

Answer 1

if only the the area is stated :
it's sure and obvious that they'll mention one of the sides. and when when one side's length and the area is mentioned then it becomes to formulate.

A=given
L/B= given
then=>

A=l*b
A=L*?
?=L/A

now, as we have both length and breadth we can formulate the perimeter. 
P=2(l+b)

-------------------------------------example----------------------------------------------------

area=10cm²
one side = 5cm
perimeter= ?

area= l*b
10=5*x
x=10/5
x=2
b=2cm

perimeter = 2(l+b)
                =2(5+2)
                =2(7)
                =14
               
=14cm

Answer 2

2(l+b)

is the perimeter of rectangle