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a in (-oo:+oo)
2*a+3 = -2*a-9 // + -2*a-9
2*a-(-2*a)+3+9 = 0
2*a+2*a+3+9 = 0
4*a+12 = 0 // - 12
4*a = -12 // : 4
a = -12/4
a = -3
a = -3
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