Its decreasing because y is decreasing as x is increasing. If you draw out a graph, the line will go downhill as you move from left to right
This is a straight line as each time the decrease is the same. In this case, each time x increases by 1, the y drops by 6. So the slope is -6/1 = -6. The negative slope means decrease. If you picked any two points from the table, you can use the slope formula m = (y2-y1)/(x2-x1) and you'd find that m = -6 each time.
