Rate of change refers to slope so let's find the slope of the line. It begins at time 0 and ends at time 3, so we can use those as the x-coordinates in our 2 points. When x = 0, y = 150; when x = 3, y = 0. Use these coordinates in the slope formula:
which gives us a slope of -50. That slope means that for every second (x) that went by, y (whatever it is) decreases by 50.