Step-by-step explanation:
The first step to completing the square is by dividing "b" by 2, squaring that number, and adding it on both sides. In this case, 16/2 = 8 and 8 squared = 64. In answer choice B, this is correctly added to both sides of the equation.
See the full steps for completing the square below. If you're still stuck on this, I suggest watching the completing the square video on Tutors.com. It is very helpful!
Have a great day!