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x in (-oo:+oo)
Y-80 = (-21/4)*(X-10) // - (-21/4)*(X-10)
Y-((-21/4)*(X-10))-80 = 0
(21/4)*(X-10)+Y-80 = 0
21/4*(X-10)+Y-80 = 0
21/4*(X-10)+Y-80 = 0
21/4*X+Y-80-105/2 = 0
21/4*X+Y-265/2 = 0
21/4*X+Y-265/2 = 0
x belongs to the empty set
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