This is the only answer choice with a negative slope, so we don't even need to look at the point really. Though if you wanted, you can confirm that the point (1,-3) is on the diagonal line. This point is shown in red in the attached image.
To find that the slope is -6/5, you pick two points on the diagonal line and use the slope formula. Two points you can use are (1,-3) and (-4,3). The slope formula is m = (y2-y1)/(x2-x1)
Note: The negative slope is because we move downhill as we move from left to right along the diagonal line.
