Add 9 to both sides to form a perfect square trinomial:
x² + 6x = 7
x² + 6x + 9 = 16
x² + 2•x•3 + 3² = 16
(x + 3)² = 16
(recall that (a + b)² = a² + 2ab + b²)
Now solve for x. Taking square roots on both sides, we get
x + 3 = ± √16
x + 3 = ± 4
x = -3 ± 4
so the two solutions are x = -7 and x = 1.