Answer:
[tex]{ \tt{rx - st = r}}[/tex]
» Collect like terms, r terms on the left hand side by subtracting r from both sides and adding st to both sides
[tex]{ \tt{(rx - st) { \green{ - r + st }} = r{ \green{ - r + st}}}} \\ { \tt{rx - r = st}}[/tex]
» On the left hand side, factorise out r
[tex]{ \tt{ \green{r(x - 1)} = st}} \\ \\ { \boxed{ \tt{ \blue{ \: r = \frac{st}{x - 1} }}}}[/tex]
