For this case, the first thing we must do is define a variable.
We have then:
p: rate in miles per hour for the last 1.5 hours
We now write the equation that models the problem.
We have then:

Rewriting we have:

From here, we clear the value of p.
We have then:
Answer:
DeAngelo's rate for the last 1.5 hours of his run is 7 miles per hour.