[tex]8ktar=r-2y\ \ \ |-r\\\\8ktar-r=-2y\\\\r(8kta-1)=-2y\ \ \ |:(8kta-1)\neq0\\\\r=\dfrac{-2y}{8kta-1}\to r=\dfrac{2y}{1-8kta}[/tex]
Example from comment:
[tex] 8k+ar=r-2y\ \ \ |-8k\\\\ar=r-2y-8k\ \ \ |-r\\\\ar-r=-2y-8k\\\\r(a-1)=-2y-8k\ \ \ |:(a-1)\neq0\\\\r=\dfrac{-2y-8k}{a-1}\to r=\dfrac{-(2y+8k)}{-(1-a)}\to r=\dfrac{2y+8k}{1-a} [/tex]
