Answer:
a = - 3
Explanation:
r² - 6r + 9
consider the factors of the constant term (+ 9) which sum to give the coefficient of the r- term (- 6)
the factors are - 3 and - 3 , since
- 3 × - 3 = + 9 and - 3 - 3 = - 6 , then
r² - 6r + 9
= (r - 3)(r - 3)
= (r - 3)² ← in the form (r + a)²
with a = - 3