Solution
= Solution
The <determinant> of $P$ is $2\cdot1-1(-1)=3$. The two-by-two <inverse matrix> formula yields
$$
\boxed{P^{-1}=\frac13\begin{pmatrix}1&-1\\1&2\end{pmatrix}}.
$$
= Solution
The <determinant> of $P$ is $2\cdot1-1(-1)=3$. The two-by-two <inverse matrix> formula yields
$$
\boxed{P^{-1}=\frac13\begin{pmatrix}1&-1\\1&2\end{pmatrix}}.
$$