Answer:
all you need to do is to multiple
Can you please be more specific? He drank 4/5 of the bottle and if the bottle is a 2 liter bottle, he drank 1.6
-19 is the answer ////////////////// sorry i needed more charecters to answer

so... notice the picture below, the angle in the fourth quadrant
now... notice, the cosine is just the distance the angle makes with the x-axis
so... using the pythagorean theorem, get the opposite side, from that triangle, keeping in mind that, "y" is negative in the 4th quadrant
once you get "y", get any other trigonometry value you need :)