Create query method with parameter condition
There is no startDate variable in getAddedQty(). You have no such a variable declared in the method, no such a variable in the class header and also no such a variable was inherited from a parent class. Therefore the compiler is right when saying that such a variable doesn't exist.
You have it in another class - the contract class. You can access the variable directly from other classes, but it's exactly what parmStartDate() method is for.
By the way, running a separate query for every transaction would be very inefficient. You should try to create a query that calculates for all transactions at once.