In math, if you are tracking your actual weight on a month to month basis, what kind of slope would you expect (negative or positive), remembering that goal of a diet is to lose weight.
You would plot time (months) along the x-axis, and the weight along the y-axis.
Positive slope means the y-values increase with increase in x-values, and negative slope means the y-values decrease as x increases.
Since losing weight means weight decreases with time, you have a line/curve that goes downwards as time increases.
Will let you take it from here.