Ok, the slope in a graph indicates if the magnitude on the y-axis increases when the magnitude in the x-axis does.
In this case our magnitudes to work with are height and weight. A negative slope implies that the more taller a person is the less weight he/she has. We know that this is not true in general. Usually, the taller the people are the heavier they get so when height increases, weight also increases. According to what I state above the graph is expected to have a positive slope.