Answer: decrease, increase
The term "demand" refers to how much consumers desire to own a product. This will indicate how much they are willing to pay for it. An increase in price will mean that demand will decrease, because less people will desire the product enough to pay the extra amount, and viceversa (if everything else stays equal). In this case, if the price of butter increases, it will decrease the demand for it. At the same time, it will increase the demand for margarine as a substitute.
