"expr" evaluates 2 and only 2 expressions.
In order to accomplish what you want to do
using "expr" you need to nest expr's and
of course, remember to "escape" the speecial
characters. An example would be...

echo `expr \`expr 1 + 2\` \* 3`

This expression, when run at the command
prompt, should yield 9. Of course, if you
are using ksh and not planning on running
a different shell, I would stick with the
ksh built-in capabilities.