we see in the points that y changes +5 units (from 1 to 6). so, to get a negative slope, the x change must be negative then (and must be -1, so that we still get the slope of -5/1).
in order for the x change to be -1 when going from point 1 to point 2, the starting x of point 1 must be 4 + 1 = 5.
if the temperature was at 15 and dropped to zero the amount it changed would be is -15. The integer is -15 because an integer is a whole number and it can also be a negative or positive number it just can not be a fraction
