The part of the problem on the left color(red)(355-9x) is going to be in the color red. The part of the problem on the right color(blue)(229+12x) is going to be done in the color blue. The color(purple)(answer) will be in the color purple.
Lets start off with writing the problem
color(red)(355-9x)color(green)(=) color(blue)(229+12x)
First lets add color(orange)(9x) to each side.
color(red)(355-9x+) color(orange)(9x) color(green)(=) color(blue) (229+12x+) color(orange)(9x)
This will change the problem to
color(red)(355) color(green)(=) color(blue)(229+21x)
The next thing we are going to do is subtract color(orange)(229) from both sides.
color(red)(355-) color(orange)(229) color(green)(=) color(blue)(229+21x-) color(orange)(229)
This will now give you the problem
color(red)(126) color(green)(=) color(blue)(21x)
The last step is to divide both sides by color(orange)(21)
color(red)(126-:) color(orange)(21) color(green)(=) color(blue)(21x-:) color(orange)(21)
This will give you your last and final answer of
color(purple)(x=6)