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#
