Simply multiply each term in each bracket, but not terms in the same bracket.
(color(red)(2x)color(blue)(-5))(color(green)(6x^2)color(red)(-6x)color(blue)(-11))
color(red)(2x) xx color(green)(6x^2)=color(orange)(12x^3)
color(red)(2x) xx color(red)(-6x)=color(green)(-12x^2)
color(red)(2x) xx color(blue)(-11)=color(red)(-22x)
color(blue)(-5) xx color(green)(6x^2)=color(green)(-30x^3)
color(blue)(-5) xx color(red)(-6x)=color(red)(30x)
color(blue)(-5) xx color(blue)(-11)=color(blue)(55)
Collect like terms:
color(orange)(12x^3)color(green)(-12x^2-30x^2)color(red)(-22x+30x)color(blue)(+55)
rArr color(orange)(12x^3)color(green)(-42x^2)color(red)(+8x)color(blue)(+55)
rArr 12x^3-42x^2+8x+55