TeX source:
\begin{array}{rcl}\mathbb{D} &=& \mathbb{R}\times\mathbb{R} \\\\f(x,y) &=& \left(x+3y\right)^2-2xy \\&=& x^2+6xy+9y^2-2xy \\&=& x^2+4xy+9y^2\end{array}