Multiplication Using Values From Various Tables
First, excellent that you provided create table statement. For additional bonus you can provide insert statements and expected output of the query ;-) 

Second, some things are missing in your model. What is the primary key of ContainsItems, I guess it should be invoiceID,itemID and for that they need to be declared as NOT NULL. What does it mean if itemQuantity is null? I assume it can't be that (may be incorrect):

CREATE TABLE ContainsItems (
    invoiceID nvarchar(5) NOT NULL REFERENCES Invoices(invoiceID),
    itemID nvarchar(5) NOT NULL REFERENCES Items(itemID),
    itemQuantity numeric(2, 0),

    primary key (invoiceID, itemID)
);


In the same spirit I assume that an Item has a name, unit and PricePerUnit:

CREATE TABLE Items (
    itemID nvarchar(5) NOT NULL PRIMARY KEY,
    itemName nvarchar(20) NOT NULL,
    unit nvarchar(20) NOT NULL, 
    pricePerUnit float NOT NULL
);


If my assumptions are wrong you will have to adjust the query to handle this. 

What you want is the SUM of the price of items per invoice. I assume this to be:

ContainsItems.itemQuantity * Items.pricePerUnit


Your query then only needs some slight modification. First we need to add information on what to apply the ? later on:

SELECT Invoices.invoiceID, Invoices.invoiceDate, ?
FROM Invoices
JOIN ContainsItems 
    ON Invoices.invoiceID = ContainsItems.invoiceID 
JOIN Items 
    ON ContainsItems.itemID = Items.itemID

GROUP BY Invoices.invoiceID, Invoices.invoiceDate


Now per such group we can sum the ContainsItems.itemQuantity * Items.pricePerUnit

SELECT Invoices.invoiceID, Invoices.invoiceDate

     , SUM( ContainsItems.itemQuantity * Items.pricePerUnit )

FROM Invoices
JOIN ContainsItems 
    ON Invoices.invoiceID = ContainsItems.invoiceID 
JOIN Items 
    ON ContainsItems.itemID = Items.itemID

GROUP BY Invoices.invoiceID, Invoices.invoiceDate