There are a number of matrix options in \AmSTeX.

There is pmatrix.
$$
A=\pmatrix
x-\lambda & 1 & 0\\
0 & x-\lambda & 1\\
0 & 0 & x-\lambda
\endpmatrix
$$

There is bmatrix.
$$
A=\bmatrix
x-\lambda & 1 & 0\\
0 & x-\lambda & 1\\
0 & 0 & x-\lambda
\endbmatrix
$$

There is vmatrix.
$$
A=\vmatrix
x-\lambda & 1 & 0\\
0 & x-\lambda & 1\\
0 & 0 & x-\lambda
\endvmatrix
$$

There is Vmatrix.
$$
A=\Vmatrix
x-\lambda & 1 & 0\\
0 & x-\lambda & 1\\
0 & 0 & x-\lambda
\endVmatrix
$$

Justification and other formatting is available.

$$
A=\pmatrix\format \r &\quad\r\\
\cos\theta & \sin\theta\\
-\sin\theta & \cos\theta
\endpmatrix
$$

And, \AmSTeX\ supplies it own cases command.

$$
f(x)=\cases x+1, &\text{if $x>0$}\\
            x-1, &\text{if $x\le 0$}\endcases
$$

\bye