take the coefficient of the x-term, half it, then square that. add this to both sides
x² + 16x. coefficient of x is 16. half of 16 is 8. 8²=64
x² + 16x + 64 =64. this is your answer but to continue
x² + 16x + 64 = 64
(x+8)² = 64
x+8 = ✓64
x+ 8 = ±8
x = 0 or -16