To solve this question, you want to try to get the variable alone on one side.
So, your starting equation is 7a-3=3+6a. You want to subtract 6a from both sides of the equation, so that the variable a is only on the left side of the equation.
So, after subtracting, your equation looks like a-3=3. Next, you want to add 3 to both sides, to cancel out the -3 on the left side of the equation.
If performed correctly, your equation should be a=6.
Your final answer is a=6.