This comes from subtracting 4 from both sides.
a is the beggining number and n is the amount of times needed to do this. if is 17 you would need to subtract 7 from the originol number 12 (a) that many times.
4/10 is 2/5 or 0.4 in decimal form.
That's about as much as you can simplify this problem.
I hope this helps! :)