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x in (-oo:+oo)
25/35 = x/21 // - x/21
25/35-(x/21) = 0
5/7-1/21*x = 0 // - 5/7
-1/21*x = -5/7 // : -1/21
x = -5/7/(-1/21)
x = 15
x = 15
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