−10.5 = −4.9 − r
\(\begin{array}{rcll} -10.5 &=& -4.9 - r \\ -4.9 - r &=& -10.5 \qquad & | \qquad \cdot (-1) \\ 4.9 + r &=& 10.5 \qquad & | \qquad -4.9 \\ 4.9 -4.9 + r &=& 10.5 -4.9 \\ \mathbf{r} & \mathbf{=} & \mathbf{5.6} \\ \end{array}\)
http://www.cymath.com/answer.php?q=%E2%88%9210.5%20%3D%20%E2%88%924.9%20%E2%88%92%20r
