The perimeter P of a rectangle is given by the formula,

P = 2L + 2W

   = 2 (L + W)     ( Where L is the length and is W is the width of the rectangle.)                      

The perimeter P of a rectangle is

P = 2 (7 m + 12 m)                (Substitute L = 12 m and W = 7 m)

  = 2 X 19 m                            (Add : 7 + 12 = 19)

  = 38 m                                  (Multiply : 2 X 19 = 39)

Perimeter is 38 m