Hi, @Reshma,
You can go through this as an example:
def count(a, b, m, n): 
  
    # If both first and second string 
    # is empty, or if second string 
    # is empty, return 1 
    if ((m == 0 and n == 0) or n == 0): 
        return 1
  
    # If only first string is empty 
    # and second string is not empty,
    # return 0 
    if (m == 0):
        return 0
  
    # If last characters are same 
    # Recur for remaining strings by 
    # 1. considering last characters 
    #    of both strings 
    # 2. ignoring last character 
    #    of first string 
    if (a[m - 1] == b[n - 1]): 
        return (count(a, b, m - 1, n - 1) + 
                count(a, b, m - 1, n)) 
    else:
          
      
      return count(a, b, m - 1, n) 
print(count(a, b, len(a),len(b)))
Output:
4