Not sure what exactly you meant, so i solved for both possibilities...
Answer: a=-2
assuming you mean a+2/2=-1.....
a+2/2=-1
simplify..
a+1=-1
subtract 1 from both sides
a=-2
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Answer: a=-4
assuming you mean (a+2)/2=-1...
(a+2)/2=-1
simplify...
a/2+2/2=-1
simplify
a/2+1=-1
subtract 1 from both sides
a/2=-2
multiply both sides by 2
a=-4