Right now, you have y=x^2-39x-2(3x-1)^2
First, we need to expand (3x-1)^2.
To expand the brackets in the form of (color(red)(a)color(green)(x)+color(blue)(b))^2, we do (color(red)(a)color(green)(x))^2+2(color(red)(a)color(green)(x)color(blue)(b))+color(blue)(b)^2
In this case we have (color(red)(3)color(green)(x)+color(blue)(-1))^2
(3x)^2=9x^2
2*3x*-1=-6x
-1^2=1
9x^2-6x+1
But we need 2 lots, so 2(9x^2-6x+1)=18^2-12x+2
y=x^2-39x+18x^2-12x+2=19x^2-51x+2
The quadratic formula is x=(-color(blue)(b)+-sqrt(color(blue)(b)^2-4color(red)(a)color(orange)(c)))/(2color(red)(a))-=(-color(blue)(-51)+-sqrt(color(blue)(-51)^2-4(color(red)(19)*color(orange)(4))))/(2(color(red)(19)))=(color(blue)(51)+-sqrt(color(blue)2601-color(yellow)(152)))/(38)=0.04 " or " 2.644
