Answer:
length = 20cm
breadth = 8cm
Explanation:
solution;
length(l) = x + 12
breadth(b) = x
perimeter (p) = 56 cm
 we know that,
perimeter(p) = 2(l + b)
or, 56 = 2(x + 12 + x)
or, 56 = 2(2x + 12)
or, 56 = 4x +24
or, 56 - 24 = 4x
or, 32 = 4x
or, 32 / 4 = x
x = 8
now 
putting the value of x in length and breadth
length = x+ 12
 =8 + 12
 =20cm
breadth = 8cm