Negative demand shock

Hi!
I would like a little help to understand how this negative demand shock works.
I have this demand function Q=10- 1/2 p and there is a negative shock so it decreases of 20%.
Should I do 0.8(10-1/2p)? Or Should I leave out the intercept? Can someone help me to understand it? If there is some textbook where I also can find an explanation would be great.
Thank you