uniform probability distribution

Goodmorning MHF!

New day, new questions...

Thanks in advance for all the help you are giving me.

Now the question:

There is a stretch of river 4 miles long. Uniform continuos distribution.

X denotes the distance in miles of an emergency from the point zero of this stretch of river. The density function is

.25 for 0<x<4

0 otherwise

The rescue team base is at the midpoint of this stretch of river. Find the probability that a given emergency arises more than 1.5 miles away from this base.

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I did F(4)- F(3.5) and then I thought I could square becasue the emergency can happen from one side or the other independently. But the result is not that on the book.:(

Thank you a lot.