Solution,

               Let the length and breadth of a rectangular park be x

Therefore-. L=5x and B = 2x

Now,

         Perimeter=2(l b)

       or,. 168m = 2(5x 2x)

       or,. 168m = 2(7x)

       or,. 168m = 14x 

       or,. 168/14 = x

       or,. 12m =x

       Therefore,

                         X=12m

Putting the value of x in 

L= 5x

   =5×12

    =60m ANS

B=2x 

 = 2×12

 = 24m ANS