GROVE PRELIM 2U/3U EX 8.9 - Q8

$y'=2(x+3)^2+2x\times 2(x+3)$

$y'=2(x^2+6x+9)+4x^2+12x$

$y'=2x^2+12x+18+4x^2+12x$

$y'=6x^2+24x+18$

$\therefore 6x^2+24x+18=14$

$6x^2+24x+4=0$

$3x^2+12x+2=0$

$x=\displaystyle { -12\pm \sqrt{12^2-4(3)(2)} \over 6}$

$x=\displaystyle { -12\pm \sqrt{120} \over 6}$

$x=\displaystyle { -12\pm 2\sqrt{30} \over 6}$

$x=\displaystyle { -6\pm \sqrt{30} \over 3}$