The grocery store is located at a point on a coordinate plane with the same y-coordinate as the bank but with the opposite x-coordinate. The grocery store and bank are reflections of each other across which axis?

Across the y axis. This question becomes easier if you graph it out in your head. Let's say the bank is at (2,2). Because the grocery store has an opposite x coordinate, the bank would be at (-2,2). If you look at this in your head, they are not even on different sides of the x axis, and therefore must be reflected across the y axis.