I know that the value of the unshaded part is probably 6,because the entire area is 45.5x9 is 45,and 3=6 makes up 9,so the value of the unshaded part is 6.I dont really understand part b though.
The answer is Division property of equality. This is because at Step 3, you have to divide to get your final answer. It cannot be the other ones because you do not subtract or distribute in step 3.