Find the directional derivative of the given function at the indicated point in the given direction.

a. \(f(x, y)=x^{2}-y^{2},(3,2), \mathbf{u}=\mathbf{i}+\mathbf{j}\).

b. \(f(x, y)=\frac{y}{x},(2,1), \mathbf{u}=3 \mathbf{i}+4 \mathbf{j}\)

c. \(f(x, y, z)=x^{2}+y^{2}+z^{2},(1,0,2), \mathbf{u}=2 \mathbf{i}-\mathbf{j}\).