What is -3x + 15 = equal to

On a piece of paper graph f(x)=5•(0.4)^x

Abby is preparing fruit punch for her party. she starts with a bowl filled with liquid that has 1/2 fruit juice and 1/2 sprite, but she decides it's too fruity. so she pours out 1/4 of the liquid, and then fills the bowl up again with sprite. what fraction of the punch is now sprite?

Express the following as a function of a single angle. cos(60) cos(-20) - sin(60) sin(-20)