-21-3s=-17-12s
-3s+12s=-17+21
9s=4
s=\frac{4}{9}
spr.
-21-3*\frac{4}{9}=-17-12*\frac{4}{9}
-21-\frac{3*4}{9}=-17-\frac{12*4}{9}
-21-\frac{4}{3}=-17-\frac{48}{9}
-21-1\frac{1}{3}=-17-5\frac{3}{9}
-22\frac{1}{3}=-22\frac{1}{3}
…
18-8y=5y-9
-8y-5y=-9-18
-13y=-27
y=\frac{-27}{-13}=2\frac{1}{13}
spr.
18-8*2\frac{1}{13}=5*2\frac{1}{13}-9
18-8*\frac{27}{13}=5*\frac{27}{13}-9
18-\frac{8*27}{13}=\frac{5*27}{13}-9
18-\frac{216}{13}=\frac{135}{13}-9
18-16\frac{8}{13}=10\frac{5}{13}-9
1\frac{5}{13}=1\frac{5}{13}