A word processor that was purchased for $600

depreciates $100 per year. What is the slope of the graph?

The slope of the graph shows the rate of change, or, in this case, the rate of depreciation, which you are told is $100 per year. So, depending upon the units used in your graph, the slope will be 100/1, and since it is a decrease as the years increase, it will have a negative slope, i. e. - 100/1.

Or, if you had drawn your graph with units of $100 dollars on the y-axis, then you might say that the slope was - 1 / 1.

The corresponding formula would of course be

y = 600 - 100t

where y = value after t years, and (if you are into calculus) differentiating would give you

slope = dy/dt = - 100

as noted above.