Expr issues

I have two variables that are set by a number i.e.

recs_on_db=12
yesterdays_recs_on_db=10

i need to get the difference between the two and be able to set this in a variable

I have tried expr but does not work:

db_difference=`expr $recs_on_db - $yesterdays_recs_on_db`

i get:

expr: syntax error

i want it to set variable $db_difference with the difference

2

have solved it! thanks

... and the verdict was?