Answer:
10 miles 
Step-by-step explanation:
This is a right triangle. This is know because of the little square in the corner that indicates a right angle. Therefore, the Pythagorean Theorem can be used. 
a^2+b^2=c^2 
where a and b are the legs and c is the hypotenuse. 
In this triangle, 24 and x are the legs, because they form the right angle. 26 is the hypotenuse because it is opposite the right angle. 
x^2+24^2=26^2 
Evaluate the exponents 
x^2+576=676
Now, solve for x. First, move 576 to the other side of the equation by subtracting 576 from both sides. 
x^2+576-576=676-576
x^2=100 
Now, x is being squared. Take the square root of both sides. 
√x^2=√100
x=√100
x=10
Use appropriate units 
x=10 miles 
The missing side length is 10 miles.