First, expand the terms in parenthesis by multiplying the terms within the parenthesis by the term outside the parenthesis:
#3x + color(red)(3)(x - 2) - 12 = -24#
#3x + (color(red)(3) * x) - (color(red)(3) * 2) - 12 = -24#
#3x + 3x - 6 - 12 = -24#
#(3 + 3)x + (-6 - 12) = -24#
#6x + (-18) = -24#
#6x - 18 = -24#
Next, add #color(red)(18)# to each side of the equation to isolate the #x# term while keeping the equation balanced:
#6x - 18 + color(red)(18) = -24 + color(red)(18)#
#6x - 0 = -6#
#6x = -6#
Now, divide each side of the equation by #color(red)(6)# to solve for #x# while keeping the equation balanced:
#(6x)/color(red)(6) = -6/color(red)(6)#
#(color(red)(cancel(color(black)(6)))x)/cancel(color(red)(6)) = -1#
#x = -1#