Answer:
The road running along the third side was 3 miles long.
Step-by-step explanation:
Given,
Length of hypotenuse = 
Length of one leg = 
We have to find the length of the third side.
Solution,
Since the field is in the shape of a right triangle.
So we use the Pythagoras theorem to find the length of the third side.
"In a right angled triangle the square of the hypotenuse is equal to the sum of the squares of other two sides".
On framing in equation form, we get;

Where 'a' = 1st side of right triangle
'b' = 2nd side of right triangle
'c' = hypotenuse
Now we put the given values and get;

Now taking square root on both side, we get;

Hence The road running along the third side was 3 miles long.