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m in (-oo:+oo)
-7*m = -21/4 // + -21/4
-7*m-(-21/4) = 0
21/4-7*m = 0
21/4-7*m = 0 // - 21/4
-7*m = -21/4 // : -7
m = -21/4/(-7)
m = 3/4
m = 3/4
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