In a graph where position is the dependent variable and time is the independent, the slope of the graph (or the slope of the tangent at a point if the graph is curved) represents the velocity. The absolute value of velocity is speed.
First let's calculate the slope of the graph at t = 2.5.
slope = change in y/change in x
change in x is 4s - 1s = 3
change in y is 1m - 5m = -4
slope = -4/3
Therefore, the velocity is -4/3 m/s, so the speed is 4/3 m/s.