Answer:
Slope = -1 
Step-by-step explanation:
Demand is buyers ability & willingness to buy at a price, time.
Demand Curve is graphical representation of quantity demanded at various prices at y axis, demand at x axis. 
Slope = Change in Y i.e ∆Y / Change in X i.e ∆X 
'Slope of Demand Curve' is a varied version of 'Price Elasticity of Demand' i.e quantity demanded responsiveness to change in price. Former shows relative change in quantity demanded over a change in price & latter shows change in price for a given change in quantity demanded. 
Demand Curve Price at Y axis, Quantity at Axis, Slope= ∆Y/∆X becomes 
= ∆P/∆Q. As per given details, ∆P/∆Q = (9-10)/(5-4) = -1/1 = -1