To find the length of the unknown side \( x \) in the area of the cross section, we can use the formula for the area of a rectangle:
\[ \text{Area} = \text{Length} \times \text{Width} \]
In this case, the dimensions of the rectangle are 12 yd, \( x \) yd, and 21 yd. Given that the area is 168 square yards, we can set up the equation as follows:
\[ 12 \times x = 168 \]
To solve for \( x \), we can divide both sides of the equation by 12:
\[ x = \frac{168}{12} = 14 \]
Thus, the length of the unknown side is
\[ \boxed{14} \text{ yd} \]