Answer? At this time -a=2, there are:

Answer? -a-2=0: (a-2)(a+ 1)=0

Solution: a=2 or a=- 1.

When a=2, element 2 in set A is duplicated, so it should be discarded, and a=- 1 is obtained.

Or:

Answer? At this time -a=a:

Answer? -2a=0: a(a-2)=0

Solution: a=2 (truncation) or a=0.

To sum up, a=- 1 or 0.