Assume that the distance left to drive is x
We know that the total distance from Melbourne to Pensacola is 500 miles
This distance is equivalent to the summation on the distance already covered (240 miles) and the distance left to drive (x)
This means that:
x + 240 = 500
Now, we solve for x:
x + 240 = 500
x = 500 - 240 = 260
Louella still needs to drive 260 miles to reach Pensacola
Hope this helps :)