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y in (-oo:+oo)
(-4/21)*y = -8/15 // + -8/15
(-4/21)*y-(-8/15) = 0
(-4/21)*y+8/15 = 0
8/15-4/21*y = 0 // - 8/15
-4/21*y = -8/15 // : -4/21
y = -8/15/(-4/21)
y = 14/5
y = 14/5
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