First, expand the term in parenthesis by multiplying each term within the parenthesis by the term outside the parenthesis:
#-6 = color(red)(9)(x + 8) + 3x#
#-6 = (color(red)(9) xx x) + (color(red)(9) xx 8) + 3x#
#-6 = 9x + 72 + 3x#
Next, group and combine like terms on the right side of the equation:
#-6 = 9x + 3x + 72#
#-6 = (9 + 3)x + 72#
#-6 = 12x + 72#
Then, subtract #color(red)(72)# from each side of the equation to isolate the #x# term while keeping the equation balanced:
#-6 - color(red)(72) = 12x + 72 - color(red)(72)#
#-78 = 12x + 0#
#-78 = 12x#
Now, divide each side of the equation by #color(red)(12)# to solve for #x# while keeping the equation balanced:
#-78/color(red)(12) = (12x)/color(red)(12)#
#-(6 xx 13)/color(red)(6 xx 2) = (color(red)(cancel(color(black)(12)))x)/cancel(color(red)(12))#
#-(color(red)(cancel(color(black)(6))) xx 13)/color(red)(color(black)(cancel(color(red)(6))) xx 2) = x#
#-13/2 = x#
#x = -13/2#