cos z = x/10 is better. try with that
(by the way, plugging in 8 right away is bad. not the right thing to do. if something is changing, you don't plug in a constant to begin with, because it will have a rate that shows up upon implicit differentiation. use variables to represent things that change. only plug in values if they are constant and/or you want to solve for something after doing all your differentiation and mumbo jumbo. also, per the question, it would be x = 8 as opposed to y = 8 for the instant in question).