Let f(x) = 2x³ - 48x² + 378x + 18 . Find the minimal value of f(x) on the interval 5 ≤ x ≤ 8 .

a) 324 and occurs at x = 6
b) 250 and occurs at x = 7
c) 400 and occurs at x = 5
d) 200 and occurs at x = 8