1) 
  (a) Each party multiplies its local share by 2. Then, they send their shares to each other, allowing everyone to reconstruct the result.
  (b) The n-th player multiplies its local share by 2, but then also adds 100 to it. Now when everybody combines their shares with this incorrect share, the result will be $2a + 100$.

2) The protocol is not secure because A and B know the values of $a$ and $b$, respectively. Later, during the online phase, they will see $x-a$ and $y-b$, and can thus recover $x$ and $y$.