First, add and subtract the necessary values from each side of the equation to isolate the #x# terms on one side of the equation and the constants on the other side of the equation while keeping the equation balanced:
#3x + 15 - color(red)(3x) + color(blue)(24) = 8x - 24- color(red)(3x) + color(blue)(24)#
Now group and combine like terms:
#3x - color(red)(3x) + 15 + color(blue)(24) = 8x - color(red)(3x) - 24 + color(blue)(24)#
#0 +15 + 24 = 8x - 3x - 0#
#39 = 5x#
Now, divide each side of the equation by #color(red)(5)# to solve for #x# and keep the equation balanced:
#39/color(red)(5) = (5x)/color(red)(5)#
#39/5 = (color(red)(cancel(color(black)(5)))x)/cancel(color(red)(5))#
#39/5 = x#
#x = 39/5#
or
#x = 7.8#