y - 29 = x² - 8x ←I moved the constant to the other side
y - 29 + (-4)² = x² - 8x + (-4)² ← I just completed the square (b/2)² added to each side
y - 13 = (x - 4)² ← factored perfect square trinomial
y = (x - 4)² + 13 ← move constant back with binomial