The length and breadth of a rectangle are in the ratio 3 :1 . If the breadth is 07 cm then the length of the rectangle is:

One side in 3 cm and other is 1 cm, Simply there is ratio of 03 :01
There are two measures as Length and Width.
Length will be written first in all measures.
03 is length and 01 is width
Width = 01 cm
Length = width * 03 = 1*3=03 cm
Width = 07 cm
Length = width * 3 = 21cm