Answer:
third option
Explanation:
Given
f(x) = 40x + 5x² ← express in standard form
f(x) = 5x² + 40x ← factor out 5 from each term
f(x) = 5(x² + 8x)
To complete the square
add/ subtract ( half the coefficient of the x- term)² to x² + 8x
f(x) = 5(x² + 2(4)x + 16 - 16)
f(x) = 5(x + 4)² + 5(- 16)
f(x) = 5(x + 4)² - 80