11.5.4.15 With constant coefficients

Problem: Bring the second order linear partial differential equation to canonical form. \[ \frac{\theta^{2} u}{\theta x^{2}}-2 \frac{\theta^{2} u}{\theta x \theta y}+\frac{\theta^{2} u}{\theta y^{2}}+\frac{\theta u}{\theta x}+\frac{\theta u}{\theta y}+u=0 \]