P(1≤x≤3) defined the probability that the result is between 1 and 3, included. You can answer this question in two ways:
1. Sum the probabilities of good events:
From the graph, we have

So,

2. Use complementary probabilities
Asking that the result is 1, 2 or 3 is the same as asking that the result is not 4. The probability that the result is 4 is 0.16, so the probability that the result is not 4 will be

The result is obviously the same.
The coefficient is 126 because 9 times 14 is 126
The y-intercept is where the curve crosses through the y axis (it intercepts it). In this case the y intercept would be worth three dollars at year 0. As years are time, and time is generally placed along the x axis.