Use any letter "x" is the usual one but since you have 2 you can use 2 different ones.
Simplify the integrand:

Now integrate one term at a time.



Putting everything together,

The subtraction of 4 on both sides would isolate the term 1.1j, then the division of 1.1 on both sides would isolate j.
Answer:
48 mph
Step-by-step explanation: Add 42mph and 54 mph and you get 96. To find the average of the two --- divide 96 by 2 and you get 48mph as the average speed for the entire trip.