Explanation:
m - 6 = - 3
send - 6 to the other side
m = - 3 + 6
when you send something to the other side it gets the opposite sign. If it was a minus sign it will be plus sign or if it was a minus sign it will be a plus sign. So - 6 became +6 when we send it to right side of the equation.
Now we need to sum - 3 +6
m = - 3 +6
m = + 3
You may not need the plus sign and you can directly write m = 3
Hope it was clear and it helps! Please let me know if you have any questions.