Compute the slope of linear demand function.

Yes, Krishna, -b = (Q^d - a)/P is the slope. I've computed it below using the graph.

The function of linear demand function is,

Q^d = a - bP where Q^d is the quantity demanded, a, b > 0, and P is the price.

-b is the slope. The negative represents if the price increases then quantity demand decreases.

To compute slope,
(P₁, $Q_1^d$) is point of departure
.(P₂, $Q_2^d$) is point of arrival.

Slope = $\frac{Q_2^d \, - \, Q_1^d}{P_2 \, - \, P_1}$ = $\frac{+ve}{-ve}$ = negative result

where, P₁ > P₂