If the question is only asking for the difference between the y in the two points, then the difference can be simply calculated as y2 - y1 as follows:
∆y = y2 - y2 = 10-(-5) = 15 units
If the two given points are to be used to determine the slope of a line, then the slope can be calculated as follows:
slope = (y2-y1) / (x2-x1) = (10--5) / (0--3) = 15/3 = 5
