We have a linear model, that relates temperature with altitude.
The first senctence gives us a clue about tha rate of change (the slope) of temperature in function of the altitude.
We can write the slope, in °F/1000 ft, as:
The temperature on the ground, where h=0, is 56.9 °F, so we can write the equation of the line as:
When the altitude is 8,000 ft, h=8 and T is:
The temperature at 8,000 ft will be 47.3 °F.