If a - b = 3 and a^2 + b^2 = 31, find the value of ab.
Choose one:
A) 12  B) 15  C) 10  D) 11  E) 13

Rationale: "2 ab = ( a ^ 2 + b ^ 2 ) - ( a - b ) ^ 2 = 31 - 9 = 22 ab = 11 . answer is d ."
Answer: D