If \(u = {\tan ^{ - 1}}\left( {\frac{{{y^3} - {x^3}}}{{{x^2} + {y^2} + xy}}} \right)\) the value of \(x\frac{{\partial u}}{{\partial x}} + y\frac{{\partial u}}{{\partial y}}\) at x = 1 and y = 3 is _____