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x in (-oo:+oo)
y = (6/7)*x-21 // - (6/7)*x-21
y-((6/7)*x)+21 = 0
(-6/7)*x+y+21 = 0
y-6/7*x+21 = 0 // - y+21
-6/7*x = -(y+21) // : -6/7
x = (-(y+21))/(-6/7)
x = 7/6*(y+21)
x = 7/6*(y+21)
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