Top//Answer-25B-1

Answer to Question 25B-1

Here are the equations for reference. 
(length.1) length([]) = 0
(length.2) length(L) = 1 + length(tail(L))
Evaluation is as follows. 
  length([6,5,4,3])
    = 1 + length([5,4,3])                by (length.2)
    = 1 + (1 + length([4,3]))            by (length.2)
    = 1 + (1 + (1 + length([3])))        by (length.2)
    = 1 + (1 + (1 + (1 + length([]))))   by (length.2)
    = 1 + (1 + (1 + (1 + 0)))            by (length.1)
    = 4