A wave front is represented by the plane y=3–x. The propagation of wave takes place at

Wave front and direction of propagation are perpendicular to each other.

So product of the slope of the two lines will be -1

$$\begin{gathered} \mathrm{equation} \mathrm{of} \mathrm{the} \mathrm{wavefront} \mathrm{is} \\ \mathrm{y}=3-\mathrm{x} \mathrm{slope} \mathrm{is} -1 \end{gathered}$$

$$\begin{gathered} \mathrm{so} \mathrm{direction} \mathrm{of} \mathrm{propagation} \mathrm{should} \mathrm{have} \mathrm{a} \mathrm{slope} \mathrm{of} 1 \\ \mathrm{i}.\mathrm{e} \mathrm{tan\theta }=1 \mathrm{so} \mathrm{\theta }={45}^0 \end{gathered}$$