Ex-2.38

Result from fold-left and fold-right is same only when op is such that (op x y) is equal to (op y x), mathematically its called the commutative property.

Ex-2.37

(define (matrix-*-vector m v)
  (map
   (lambda (w)
     (dot-product v w)) m))
(define (transpose mat)
  (accumulate-n
   (lambda (x y) (cons x y))
   '() mat))
(define (matrix-*-matrix m n)
  (let ((cols (transpose n)))
    (map
     (lambda (v)
       (map (lambda (w) (dot-product v w)) cols))
     m)))

Ex-2.36

(define (accumulate-n op init seqs)
  (if (or (null? seqs) (null? (car seqs)))
      '()
      (cons (accumulate op init
                        (map (lambda (x) (car x)) seqs))
            (accumulate-n op init
                          (map (lambda (x) (cdr x)) seqs)))))

Ex-2.35

basically with map we create a flat list containing number of leaves in each subtree and then accumulate is used to sum them up.

(define (count-leaves t)
  (accumulate
   + 0 
   (map (lambda (x)
          (if (pair? x)
              (count-leaves x) 1)) t))) 

Ex-2.34

(define (horner-eval x coefficient-sequence)
  (accumulate (lambda (this-coeff higher-terms)
                (+ this-coeff (* higher-terms x)))
              0
              coefficient-sequence))

Ex-2.33

(define (map p sequence)
  (accumulate (lambda (x y) (cons (p x) y)) nil sequence))
(define (append seq1 seq2)
  (accumulate cons seq2 seq1))
(define (length sequence)
  (accumulate (lambda (x y) (+ y 1)) 0 sequence)) 

Ex-2.32

(define (subsets s)
  (if (null? s)
      (list '())
      (let ((rest (subsets (cdr s))))
        (append rest
                (map (lambda (x)
                       (cons (car s) x)) rest)))))

Why it works?
Wishful thinking :)
we assumed that we got subsets of (cdr s), now all we need to think is how to get additional subsets of s that contain (car s) and the answer is simply to include (car s) in all the subsets of (cdr s).