Đáp án :
$a/$
`A (x) = 6x^3 - x (x + 2) + 4 (x + 3)`
`-> A (x) = 6x^3 - x^2 - 2x + 4x + 12`
`-> A (x) = 6x^3 - x^2 + (-2x + 4x) + 12`
`-> A (x) = 6x^3 - x^2 + 2x + 12`
$\\$
`B (x) = -x (x + 1) - (4 - 3x) + x^2 (x - 2)`
`-> B (x)= -x^2 - x - 4 + 3x + x^3 - 2x^2`
`-> B (x) = (-x^2 - 2x^2) + (-x + 3x) - 4 + x^3`
`-> B (x) = -3x^2 + 2x - 4 + x^3`
`-> B (x) = x^3 - 3x^2 + 2x - 4`
$\\$
$\\$
`C (x) = A (x) + B (x) - x^2 (7x - 4)`
`-> C (x) = A (x) + B (x) - 7x^3+ 4x^2`
`-> C (x) = 6x^3 - x^2 + 2x + 12 + x^3 - 3x^2 + 2x - 4 - 7x^3 + 4x^2`
`-> C (x) = (6x^3 + x^3 - 7x^3) + (-x^2 - 3x^2 + 4x^2) + (2x + 2x) + (12 - 4)`
`-> C (x) = 4x + 8`
$\\$
Cho `C (x) = 0`
`-> 4x + 8 = 0`
`-> 4x = -8`
`-> x = -2`
