There's two ways I can see someone doing this:
The first way requires you to work out the price of 1 night, and multiply that price by 7. So you would do 261 ÷ 3 = 87 (price) and 3 ÷ 3 = 1 (nights). So now you know that per night, it costs 87. Multiply this by 7 to get 609 as the total for 7 nights.
The second method requires you to work out 7 ÷ 3, which is 2.33333333... Multiply 261 by 2.33333333 to work out how much it is for 7 nights. (Also 609).
The first method however is a lot simpler and sometimes more accurate, because as we found in this example, we didn't encounter any decimals to get the answer.