problem: Define the operation $\star$ by $a \star b = (a + b)b$. What is $(3\star5) - (5\star3)$?
solution: Since $3 \star 5 = (3 + 5)5 = 8\cdot 5 = 40$ and $5 \star 3 = (5 + 3)3 = 8\cdot 3 = 24$, we have \[
  3\star5 - 5\star3 = 40 - 24 = \boxed{16}.
  \]