Answer:
15
Step-by-step explanation:
Rate of change or slope or gradient of a line passes two points (x1, y1) and (x2, y2) could be calculated by:
(y2 - y1)/(x2 - x1)
=> Rate of change of the line passing (1,15) and (3,45):
(45 - 15)/(3 - 1) = 30/2 = 15
