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y in (-oo:+oo)
(0.9*y)/0.7 = 4.2 // - 4.2
(0.9*y)/0.7-4.2 = 0
1.28571429*y-4.2 = 0 // + 4.2
1.28571429*y = 4.2 // : 1.28571429
y = 4.2/1.28571429
y = 3.26666666
y = 3.26666666
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