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#
