We can assume that the shape is a rectangular prism by the given dimensions
Volume of a rectangular prism
Volume = Area of Base × Height of prism
= Length × Width × Height of prism
Substitute!
168 = L × 7 × 3
168 = L × 21
Divide 21 on either sides to isolate L
= 
21 and 21 cancels out
8 = L
L = 8 yd
Volume of a rectangular prism
Volume = Area of Base × Height of prism
= Length × Width × Height of prism
Substitute!
168 = L × 7 × 3
168 = L × 21
Divide 21 on either sides to isolate L
21 and 21 cancels out
8 = L
L = 8 yd