The function f(x)=mx+b can represent y=mx+b where m represent the slope and b represent the y-intercept. In this case,
f(x)=8-2x
Or
f(x)=-2x+8
Or
y=-2x+8 where -2 represent the slope. As a result,-2 is the slope of the function f(x)=8-2x. Hope it help!