Let consider time on x-axis and temperature on y-axis. 
 The x-axis starts from “0” corresponds to “8 a.m”. Thus after 4 hours, the time will be “12pm”. So we have two values on x-axis, one is “0” and another “4”. 
 For x = 0, or time value = 8am, the temperature value = y-axis value = -2
 For x = 4, or time value = 12pm, the temperature value = y-axis value = 4
 Thus we got two points (0,-2) and (4,4). 
 Now using slope formula = (y2 –y1) / (x2 –x1)
 Slope = (4-(-2)) / (4 -0) = 6/4 = 1.5