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henaoalexandra
p=a+b+2c a=
p = a + b + 2c p - b = a + 2c a = p - b - 2c
p = a + b + 2c add -p to both sides of the equation p - p = a+b+2c-p 0=a+b+2c-p add-a to both sides of the equation -a=a-a+b+2c-p -a=0+b+2c-p multiply by (-1) both sides of the equation (-1)(-a)=(-1)(b+2c-p) a=-b-2c+p rearrange and you get: a=p-b-2c