Question

what will hapen to the perimeter of a rectangle if its length is double and breadth remains same

Answer 1

The perimeter of a rectangle is 2(l+b)
Let the length be 2l and breadth remain b
So, perimeter now becomes:
2(2l+b)

Answer 2

Let's check.
Perimeter of rectangle = 2(l+b)= 2l +2b
Now,
Perimeter of triangle by doubling length = 2(2l+b) = 4l+ 2b

i.e. there is an increment of twice the length value in perimeter.