Show that a differentiable function f decreases most rapidly....

at x in the direction opposite to the gradient vector, that is, in the direction of -Vf(x).

I made up a function of two variables f(x, y) = ln(x^2 -y^3/2) and then calculated the gradient which came to 2x/(x^2 - y^3/2) and -3/2y^2/(x^2 - y^3/2).

I then put in a value Vf (2, 1) and got the vector < 8/7 , -3/7>. I made the vector negative and got , <-8/7 , 3/7>. How do I show that a differentiable function f like the one above (or another one) decreases most rapidly at x in the direction opposite to the gradient vector?

At this point I am stuck.