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x in (-oo:+oo)
3/4-7*x = -81/4 // + -81/4
3/4-7*x-(-81/4) = 0
3/4-7*x+81/4 = 0
21-7*x = 0 // - 21
-7*x = -21 // : -7
x = -21/(-7)
x = 3
x = 3
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