The function DO computes pow(x, n), ie, x raised to n.
For X = 2, N = 37 there would be following sequence of calls:
DO (2, 37)
DO (2, 18)
DO (2, 9)
DO(2, 4)
DO(2, 2)
DO(2, 1) // end of recursion
DO(2, 1) will not have any multiplications.
For odd N there would be 2 multiplications and for even N there would be 1 multiplication.
Number of odd N's == 2 (9, 37 -> Note we exclude 1)
Number of even N's == 3 (2, 4, 18)
Therefore number of multiplications should be 2*2 + 3*1 == 4 + 3 == 7.
Please verify if that answer is correct, I did not verify it.