Given:
2 walls = 26 ft long by 9 ft wide
3rd wall = 18 ft long by 9 ft wide
Area = 26 ft * 9 ft = 234 ft²
2 walls ⇒ 234 ft² * 2 = 468 ft²
3rd wall area = 18 ft * 9 ft = 162 ft²
Total area = 468 ft² + 162 ft² = 630 ft²
630 ft² ÷ 250ft² per gallon = 2.52 gallons
Amelia should be 2.52 or 3 gallons to paint the 3 walls.