When you cut something in half: let’s say a circle ⭕️ It’s a cut between a shape that is perfectly the same measurement on each side (if you cut a circle in shape, it’s the same size and width) I hope this helps!
X² + y² - 4 x + 2 y = b x² - 4 x + 4 + y² + 2 y + 1 = b + 4 + 1 ( x - 2 )² + ( y + 1 )² = b + 5 y - coordinate of the center of the circle is: y = - 1 b + 5 = r b + 5 = 7 b = 7 - 5 b = 2