a)
[tex] \frac{t}{m} = 3 | \times m| [/tex]
[tex]t = 3m | \div 3| [/tex]
[tex]m = \frac{t}{3} [/tex]
b)
[tex]s = a + (m - 1) \times \frac{r}{2} [/tex]
[tex]s = a + \frac{mr}{2} - \frac{r}{2} | \times 2| [/tex]
[tex]2s = 2a + mr - r[/tex]
[tex]2s - 2a + r = mr | \div r| [/tex]
[tex]m = \frac{2s - 2a}{r} [/tex]
