Answer:
The direct distance from Sid´s house to school is 8.5 miles.
Explanation:
In this case we have to use a geometric law the law of Pythagoras, this could give calculate us the hypotenuse of a triangule having to known sides, this is:

Now we can replace the side a=3 miles and b=8 miles, this is:

Having this we can clear the c, and have the answer:

So the direct distance from Sid´s house to school is 8.5 miles.