First is for you to try to leave X alone on the left side to find its value.
Step 1: -3 is moved to the right side as a addition so the equation now should be 7x=21.
Step 2: We need to leave X alone so we also move 7 to the right side as a division. Your equation should look like this x=21/7, divide it and the result is x=3
Hope This Helps You
