Use Lagrange multipliers to find the dimensions of a rectangular box

So you want to maximize $f(x, y, z) = xyz$ with the constraint that $g(x, y, z) = 4x + 4y + 4z = c$.
Now use $<\frac{\delta f}{\delta x}, \frac{\delta f}{\delta y}, \frac{\delta f}{\delta z}> = \lambda<\frac{\delta g}{\delta x}, \frac{\delta g}{\delta y}, \frac{\delta g}{\delta z}>$