Let's first divide both sides of the equation by -1 to obtain:
a² + 10a - 25 = -25
To complete the square, we take 1/2 of the coefficient of a, which is 10.
1/2 of 10 is 5.
Next, we square this.
5² = 25
To complete the square, we add 25 to both sides of the equation.
(a² + 10a + 25) - 25 = -25 + 25
(a + 5)² - 25 = 0
(a + 5)² = 25
a + 5 = ± 5
a + 5 = 5
a = 0
and
a + 5 = -5
a = -10
The two solutions are 0 and -10.
