We always complete the square of a monic quadratic (coefficient on x² is one) by adding half the linear coefficient squared. Here that's half of 10, so 5, squared, so 25.
Answer: 25
x² + 10x = 5
x² + 10x + 25 = 5 + 25
(x + 5)² = 30
x + 5 = ±√30
x = -5 ±√30
