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a in (-oo:+oo)
1-4*a = 6*a-99 // - 6*a-99
1-4*a-(6*a)+99 = 0
1-4*a-6*a+99 = 0
100-10*a = 0 // - 100
-10*a = -100 // : -10
a = -100/(-10)
a = 10
a = 10
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