Answer:
see explanation
Step-by-step explanation:
(s • t)(x) = s(x) × t(x)
= (x + 4)(2x - 5) ← expand using FOIL
= 2x² - 5x + 8x - 20 ← collect like terms
= 2x² + 3x - 20
(s - t)(x) = s(x) - t(x)
= x + 4 - (2x - 5) ← distribute by - 1
= x + 4 - 2x + 5 ← collect like terms
= - x + 9 = 9 - x
(s + t)(x) = s(x) + t(x)
= x + 4 + 2x - 5 = 3x - 1
To evaluate (s + t)(4) substitute x = 4 into (s + t)(x)
= ( 3 × 4) - 1 = 12 - 1 = 11
