To calculate the final price of the textbook, you need to add the tax to the original price. The original price of the textbook is $40. Since the tax is 10 units (which we can assume is also in dollars), you simply add that to the original price.
So the calculation will look like this:
Final Price = Original Price + Tax
Final Price = $40 + $10 = $50
Therefore, the final price of the textbook after including tax will be $50.