Verified answer

a - b = 3i + 2j - k - (2i  - j + 4k) = 3i - 2i + 2j + j - k - 4k = i + 3j - 5k

b(a-b) = (1; - 1; 4) * (1; 3; -5) = 1 - 3 - 20 = -22

3b(a-b) = 3 * (-22) = -66