We can think of this as a right triangle, where the given lengths are the legs.
So, we can solve using the Pythagorean Theorem.
21² + 28² = c²
441 + 784 = c²
1225 = c²
35 = c
35 miles
Answer:

Step-by-step explanation:
we know that
When we compare positive fractions with the same numerator, the largest rational number will be the fraction with the lowest denominator
In this problem we have
and 
Both numerator are equal
Compare the denominators

therefore
is the largest rational number