basic.-

x = x - 1
Subtracts "1" from the contents of variable "x".

x = x + (-z)
Negates the contents of "z" and adds to the contents of 
"x".  The result is placed in "x".

amount = price - discount.amount
Subtracts the contents of the variable "discount.amount" from the 
variable "price".  Places the result in the variable 
"amount".

amount = (unit.cost * qty) - discount.amount
Subtracts the contents of variable "discount.amount" from the product 
of "unit.cost" and "qty".  The parenthesis "()" 
used to group precedence are unnecessary due to default precedence. They are 
added for program clarity.

counter = -1
This assigns the value, "-1", to the variable "counter".