The equation $\mathrm{I}\mathrm{I}\backslash mathrm\{II\}$ is already solved for $yy$. Therefore, the substitution method is advisable.

The other two methods also get you to the goal, but they require further transformation steps.

All three methods can be applied directly and are useful for this reason.

Equation $\mathrm{I}\backslash mathrm\{I\}$ contains $-2b-2b$, equation $\mathrm{I}\mathrm{I}\backslash mathrm\{II\}$ contains $2b2b$. So the $bb$ will drop out when adding both lines, which makes the addition method suitable.

With the other two procedures, you have to take further transformation steps, before one can apply them.