Answer:
An acute triangle, because

Explanation:
Given:
The distance between city A and city B is 22 miles.
The distance between city B and city C is 54 miles.
The distance between city A and city C is 51 miles.
thus we have a triangle ABC with side lengths :
AB = 22 miles (shortest side)
BC= 54 miles (longest side)
AC = 51 miles (shorter side)
For an obtuse triangle :

For acute triangle :

Comparing sum of squares of shorter sides with the square of longest side:


We see that:

Thus, the condition
fulfills proving this triangle to be an acute triangle.