The first step is to expand the terms in parenthesis by multiplying each term within the parenthesis by #color(red)(-3)# being careful to ensure the sign of the product is correct:

#(color(red)(-3) * x) + (color(red)(-3) * - 1) + 9 = 15#

#-3x + 3 + 9 = 15#

Next we can combine the like terms on the left side of the equation:

#-3x + (3 + 9) = 15#

#-3x + 12 = 15#

Then, we can subtract #color(blue)(12)# from each side of the equation to isolate the #x# term and keep the equation balanced:

#-3x + 12 -color(blue)(12) = 15 - color(blue)(12)#

#-3x + 0 = 3#

#-3x = 3#

Now, we can divide each side of the equation by #color(green)(-3)# to solve for #x# and keep the equation balanced:

#(-3x)/color(green)(-3) = 3/color(green)(-3)#

#(color(green)(cancel(color(black)(-3)))x)/cancel(color(green)(-3)) = -1#

#x = -1#