At 5 a. m., the temperature at an airport was −9.4°f . six hours later, the temperature was 2.8°f . which expression represents the total temperature change over the six hours?

Let x = the number of hours since 5 a.m.
Let y = temperature, °F

Let the total temperature change be modeled by the expression
y =  ax + b

At 5 a.m., x = 0 and y = -9.4°F. Therefore
b = -9.4

When x = 6 hours, y = 2.8°F. Therefore
6a + b = 2.8
6a - 9.4 = 2.8
6a = 12.2
a = 2.0333

The expression is
y = 2.0333x - 9.4

y = 2.0333x - 9.4
where
x = hours since 5 a.m.
y = temperature, °F