[tex]k-\dfrac{m}{r}=q\qquad|+\dfrac{m}{r}\\\\\boxed{k=q+\dfrac{m}{r}}\\--------------------\\\\\dfrac{k-m}{r}=q\qquad|\cdot r\neq0\\\\k-m=qr\qquad|+m\\\\\boxed{k=qr+m}[/tex]
