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a in (-oo:+oo)
1 = 7-3*a-(3*a) // - 7-3*a-(3*a)
3*a-(-3*a)-7+1 = 0
3*a+3*a-7+1 = 0
6*a-6 = 0 // + 6
6*a = 6 // : 6
a = 6/6
a = 1
a = 1
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