If b is Billy's age *now*, part A would look something like this:
b + 8
Usually, it looks a little more like x = b + 8, just because we like setting the equation to an unknown variable (usually x).
b + 8 = 21
To simplify the above equation, subtract 8 from both sides:
b = 21 - 8
OR
b = 13
We just found the answer to part C above.
I'm not going to work out part D for you (you got this), but use the subtraction sign to your benefit and replace it with a word like "ago", if you're choosing age as your word problem. You could also do the traditional candy word problem where you once had 10 jellybeans and now you have 6.... Up to you. "A" will be your "now" in that case. Have fun!