Question

X (year)| y ($ in billions)

Answer 1

Rate of change is also another name for the slope so in this case, the question is asking you for the slope.

How can we find the slope?
The formula for slope is y2-y1/x2-x1

Now let's plug in for the year 1980 and 1990.

239-120/1990-1980
= 119/10

Slope is 119/10.