How do i find the perimeter of a [__] only having the a=25m2 and w=2.5 m

-- You know the area of a rectangle, and you know its width.

Let's pause here, and gather up a few tools to do the job with.
The tools you'll need are the things that you know about rectangles.
Definitions of the area and perimeter of a rectangle would be very helpful.

=> Area of a rectangle  =  (length) x (width)

=> Perimeter of a rectangle  =
                                           (length+width + length+width) = 2 (length+width)

You know all the numbers in both of these formulas, except the length.
Can you think of a way to find it, using the numbers that you do know ?

Take the area formula . . . . .  Area  =  (L) times (W)

Put in the numbers you know:    25 m² = (L) x (2.5 m)

Divide each side by  2.5 m :        (25 m²) / (2.5 m) = L

                                                                 10 m  =  L

Now you know both  'L'  and  'W'.
so ...
Time to go directly to the perimeter formula:         

                                              Perimeter = 2 (L + W)

                                                                = 2 (10m + 2.5m)

                                                                = 2 (12.5 m)

                                                                =  25 m

Many things that seem threatening in the dark become welcoming when we shine light on them