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a in (-oo:+oo)
7-(a/4.2) = 0
7-0.23809524*a = 0
7-0.23809524*a = 0 // - 7
-0.23809524*a = -7 // : -0.23809524
a = -7/(-0.23809524)
a = 29.39999976
a = 29.39999976
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