3s − 2t = 8
3s = 8 + 2t
$s = \frac{2}{3}t + \frac{8}{3}$
$s = \frac{2}{3}t + 2\frac{2}{3}$
 
2t = 3s − 8
t = 1,5s − 4

7z + 4q = 11
7z = 11 − 4q
$z = \frac{11}{7} - \frac{4}{7}q$
$z = 1\frac{4}{7} - \frac{4}{7}q$
 
4q = 11 − 7z
$q = \frac{11}{4} - \frac{7}{4}z$
$q = 2\frac{3}{4} - 1\frac{3}{4}z$

9r − 13s = 17
9r = 13s + 17
$r = \frac{13}{9}s + \frac{17}{9}$
$r = 1\frac{4}{9}s + 1\frac{8}{9}$
 
13s = 9r − 17
$s = \frac{9}{13}r - \frac{17}{13}$
$s = \frac{9}{13}r - 1\frac{4}{13}$

5u + 7v = 21
5u = 21 − 7v
$u = \frac{21}{5} - \frac{7}{5}v$
$u = 4\frac{1}{5} - 1\frac{2}{5}v$
 
7v = 21 − 5u
$v = 3 - \frac{5}{7}u$