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.