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k in (-oo:+oo)
28 = 5*k+5*k+8 // - 5*k+5*k+8
28-(5*k)-(5*k)-8 = 0
28-5*k-5*k-8 = 0
20-10*k = 0 // - 20
-10*k = -20 // : -10
k = -20/(-10)
k = 2
k = 2
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