Ok so I'm having a little trouble figuring out which method I am supposed to use to solve this problem.

Calculate the derivative of f(x,y)= (y-x^2)/3y along the curve parameterized by r(t)= 2t(i) - t^(1/2)(j) when t=4.

Do I put the 4 in for t and then use that as the direction and find the directional derivative, or is this a chain rule problem where I take

df/dx * dx/dt + df/dy * dy/dt? Any help would be greatly appreciated. Thanks.