a question of CE physics Force

Take direction to the right as positive.

Consider the whole system.

By F=ma,
12=(1+3)a
∴a=3ms-2

Consider the block A.

By F=ma,
12-R=1(3)
∴R=9N

計完睇下個答案有冇錯
Re-check the answer.
For block B,
9=3a
∴a=3ms-2
同前面吻合