A firm encountering economies of scale over some range of output will have a

Falling long-run average cost curve

Explanation:

When a firm encounters economies of scale, which means they are producing the maximum amount of product. When a firm experiences economies of scale, they might have to add more output to generate the input. Sometimes, doubling an input required more than doubling the output that leads to failing long-run average cost curve. The best way to encounter this problem is to find a low-cost method to manufacture goods and services.