The Completing-the-Square Method:
1. Convert x² + 6x - 4 = 0 to
x² + 6x = 4
2. Convert x² + 6x to a perfect square by adding a term:

3. Keep the equation balanced by adding the same term to the right-hand side:

4. Rewrite as a square and use the Square-Root method:
(x + 3)² = 4 + 9 = 13
and solve.