in example 3 why slope = -5

The given graph shows the temperature of water, which was kept on fire for some time, at different interval of time.

It can be observed from the graph that temperature of water at 2 p.m. and 5 p.m.is 60ºC and 45ºC respectively.

The line passes through the points A(2, 60) and B(5, 45).

Suppose the temperature of the water at 8 p.m. be yºC. Let C(8, y) represents the coordinate of point which represent the temperature of the water at 8 p.m. be yºC.

The point C lies on the line AB. The line AB and line BC represents the same line, therefore, the slope of line AB is equal to the slope of line BC.

Thus, the temperature of water will be 30ºC at 8 p.m.