$\{\begin{array}{l}2a+2b=40\\ a=b+6\end{array}$

$\{\begin{array}{l}2\cdot \left(b+6\right)+2b=40\\ a=b+6\end{array}$ $\{\begin{array}{l}2\cdot \left(b+6\right)+2b=40\\ a=b+6\end{array}$

$\{\begin{array}{l}2b+12+2b=40\\ a=b+6\end{array}$

$\{\begin{array}{l}4b+12=40\\ a=b+6\end{array}$

$\{\begin{array}{l}4b=28\\ a=b+6\end{array}$

$\{\begin{array}{l}b=7\\ a=b+6\end{array}$

$\{\begin{array}{l}b=7\\ a=7+6\end{array}$

$\{\begin{array}{l}b=7\\ a=13\end{array}$

$P=a\cdot b$

$P=7\cdot 13=91$

Odpowiedź:

Pole wynosi $91c{m}^2$ .