Given:
Rectangular prism
Length = 2.5x
Width = x
Height = 6 feet
Volume = 2,940 feet³
Volume = Length * Width * Height
2,940 ft³ = (2.5x) * x * 6ft
2,940 ft³ ÷ 6 ft = 2.5x²
490 ft² = 2.5 x²
490 ft² ÷ 2.5 = x²
196 ft² = x²
√196 ft² = √x²
14 ft = x
Width = x = 14 feet
Length = 2.5x = 2.5 * 14 ft = 35 feet