Bonjour,
[tex]7 \leq p+q \leq 9\ (1)\\
11 \leq p-q \leq 13\ (2)\\
(1)+(2):\ 18 \leq 2p \leq 22\\
\boxed{9 \leq p \leq 11}\\\\
(1):\ -9 \leq -p-q \leq -7\ (3)\\
(2):\ 11 \leq p-q \leq 13\\
(3)+(2): 2 \leq -2q \leq 6\\\\
1 \leq -q \leq 3\\\\
\boxed{-3 \leq q \leq -1}
[/tex]
