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x in (-oo:+oo)
x/8 = -21/4 // + -21/4
x/8-(-21/4) = 0
x/8+21/4 = 0
1/8*x+21/4 = 0 // - 21/4
1/8*x = -21/4 // : 1/8
x = -21/4/1/8
x = -42
x = -42
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