2х + 9/4 - х - 2/6 = 3 общ.знам.12
12*2х + 9*3 - 12х - 4 = 12*3
24х + 27 - 12х - 4 = 36
12х = 36 - 27 + 4
12х = 13
х = 13/12 = 1 1/12
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2х^2 - 3x - 4x + 21 = 2x^2 - 8x + 3x - 12 + 3
2x^2 - 2x^2 -7x + 8x - 3x = - 21 - 12 + 3
- 2x = - 30
2x = 30
x = 15