a)X^2+9x+18 b)2x^2-5x-12

$\begin{array}{l} a)\,\,\,{x^2} + 9x + 18\\ = {x^2} + 3x + 6x + 18\\ = x\left( {x + 3} \right) + 6\left( {x + 3} \right)\\ = \left( {x + 3} \right)\left( {x + 6} \right)\\ b)\,\,2{x^2} - 5x - 12\\ = \,2{x^2} - 8x + 3x - 12\\ = 2x\left( {x - 4} \right) + 3\left( {x - 4} \right)\\ = \left( {x - 4} \right)\left( {2x + 3} \right) \end{array}$

`a) x^2+9x+18`

`x^2 + 6x + 3x + 18 = 0`

`x(x+6) + 3(x+6) = 0`

`x+6 = 0` hoặc `x+3 = 0`

`x = -6` hoặc `x = -3`

`b) 2x^2-5x-12`

`2x^2 - 8x - 3x -12 = 0`

`2x(x-4) + 3(x-4) =0`

`2x + 3 = 0` hoặc `x-4 = 0`

`2x = -3` hoặc `x=4`

`x= -3/2` hoặc `x=4`