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x in (-oo:+oo)
(1/7)*(x+21)-((1/8)*(x+8)) = x+7 // - x+7
(1/7)*(x+21)-((1/8)*(x+8))-x-7 = 0
(1/7)*(x+21)+(-1/8)*(x+8)-x-7 = 0
1/7*(x+21)-1/8*(x+8)-x-7 = 0
1/7*(x+21)-1/8*(x+8)-x-7 = 0
1/56*x-x-7+2 = 0
2-55/56*x-7 = 0
-55/56*x-5 = 0
-55/56*x-5 = 0
-55/56*x-5 = 0 // + 5
-55/56*x = 5 // : -55/56
x = 5/(-55/56)
x = -56/11
x = -56/11
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