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a in (-oo:+oo)
839/21 = (3/7)*a-4*a+2/3 // - (3/7)*a-4*a+2/3
839/21-(-4*a)-((3/7)*a)-(2/3) = 0
4*a+(-3/7)*a+839/21-2/3 = 0
25/7*a+275/7 = 0 // - 275/7
25/7*a = -275/7 // : 25/7
a = -275/7/25/7
a = -11
a = -11
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