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x in (-oo:+oo)
(x-1)/7 = (4*x+3)/21 // - (4*x+3)/21
(x-1)/7-((4*x+3)/21) = 0
(x-1)/7+(-1*(4*x+3))/21 = 0
(21*(x-1))/(7*21)+(-1*7*(4*x+3))/(7*21) = 0
21*(x-1)-1*7*(4*x+3) = 0
-7*x-42 = 0
(-7*x-42)/(7*21) = 0
(-7*x-42)/(7*21) = 0 // * 7*21
-7*x-42 = 0
-7*x-42 = 0 // + 42
-7*x = 42 // : -7
x = 42/(-7)
x = -6
x = -6
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