Use axes $Ox, Oy$ which are respectively parallel and perpendicular to the inclined plane. The components of the launch velocity along these axes are $v_x=v_0 \cos(\beta-\alpha)$ and $v_y=v_0\sin(\beta-\alpha)$, while the components of gravity are $-g_x=-g\sin\alpha$ and $-g_y=-g\cos\alpha$.

The co-ordinates of the arrow at time $t$ after launch are $$x=v_x t-\frac12 g_x t^2$$ $$y=v_y t-\frac12 g_y t^2$$ When the arrow lands then $y=0$ and $t=T$ the time of flight and $x=R$ the range. So $$v_y=\frac12 g_y T$$ $$T=\frac{2v_y}{g_y}=\frac{2v_0}{g}\frac{\sin(\beta-\alpha)}{\cos\alpha}$$ and $$R=v_xT-\frac12 g_xT^2=\frac{2v_0^2}{g}\frac{\cos(\beta-\alpha)\sin(\beta-\alpha)}{\cos\alpha}-\frac{2v_0^2}{g}\frac{\sin\alpha \sin^2(\beta-\alpha)}{\cos^2\alpha}$$ $$=\frac{2v_0^2\sin(\beta-\alpha)}{g\cos^2\alpha}[\cos(\beta-\alpha)\cos\alpha-\sin(\beta-\alpha)\sin\alpha]$$ $$=\frac{2v_0^2}{g}\frac{\sin(\beta-\alpha)\cos\beta}{\cos^2\alpha} $$

To find the angle of launch for maximum range note that $$2\cos A\sin B = \sin (A+B) - \sin (A-B)$$ therefore $$2\cos\beta\sin(\beta-\alpha)=\sin(2\beta-\alpha)-\sin\alpha$$ The 2nd term is fixed because $\alpha$ is fixed, but $\beta$ can vary. The maximum value this expression can have occurs when the 1st term is $+1$, ie when $$2\beta-\alpha=\frac12\pi$$ $$2(\beta-\alpha)= \frac12 \pi-\alpha$$ That is, the maximum range is achieved when the direction of launch bisects the angle between the inclined plane and the gravity vertical. The maximum range is $$R_{max}=\frac{v_0^2}{g}\frac{(1-\sin\alpha)}{\cos^2\alpha}=\frac{v_0^2}{g}\frac{(1-\sin\alpha)}{(1-\sin\alpha)(1+\sin\alpha)}=\frac{v_0^2}{g(1+\sin\alpha)}$$

*Source : Letter to the Editor of American Journal of Physics by A P French, 1984*