Hello!
A good way to solve this is by using the Pythagorean Theorem, where the distance between the grocery store and Callie's house is the hypotenuse of the triangle.
The two legs we are given, 1.8 and 2.4 will represent a and b in the Pythagorean Theorem.
a²+b²=c²
First we have to find 1.8 and 2.4 squared.
1.8²=3.24
2.4²=5.76
If we add these answers, we will get 9. This is c². But to find just c, we will undo the square by using the square root.
√9=3.
This means that the length of a straight line from Callie's house to the grocery store is 3 miles.
I hope this helps!