You mainly need to make sure that the product has a value less than 20. That's not hard to do, since the difference can't be more than 9, and can be negative.
Pick some digits to put into the expression and see what the value is. Then make any adjustments you might need to make to reduce the sum. If you put 0 in the rightmost box, the other digits can be anything. (They just have to be different.)
