Let f(x) = 8x³ - 15x² - 6, where x >= 1.5. Find the value of (df^(-1))/(dx) at the point x = 2003 = f(7). What is the simplified fraction for the value of (df^(-1))/(dx) at the point x = 2003 = f(7)?
A) -1/3
B) 1/4
C) 3/2
D) -2/5