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m in (-oo:+oo)
(-3/7)*m-4 = (1/7)*m-40/7 // - (1/7)*m-40/7
(-3/7)*m-((1/7)*m)-(-40/7)-4 = 0
(-3/7)*m+(-1/7)*m+40/7-4 = 0
12/7-4/7*m = 0 // - 12/7
-4/7*m = -12/7 // : -4/7
m = -12/7/(-4/7)
m = 3
m = 3
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