I am getting expr syntax errors for a simple equations

Question

here is the code:

lidarachynska@MacBook-Air-Lida ~ % cd rachynska
lidarachynska@MacBook-Air-Lida rachynska % vim bin/com2
lidarachynska@MacBook-Air-Lida rachynska % chmod a+x bin/com2
lidarachynska@MacBook-Air-Lida rachynska % PATH=$PATH:~/rachynska/bin
lidarachynska@MacBook-Air-Lida rachynska % com2
expr: syntax error
expr: syntax error
expr: syntax error
expr: syntax error
expr: syntax error

I've tried two ways of writing expr in com2, but both of them didn't work

No1

a=`expr $3`
b=`expr $2 - $1`
c=`expr $2 + $1`
d=`expr $b \* $c`
a=`expr $d / 7`
echo $a

No2

a=$(expr $3)
b=$(expr $2 - $1)
c=$(expr $2 + $1)
d=$(expr $b \* $c)
a=$(expr $d / 7)
echo $a