To find the length of the missing side of a shape, given the perimeter, we first note the expression for the perimeter:
\[ P = 21x^2 - 5x + 4 \]
Assuming the shape is a polygon and we know the lengths of all sides except one, we can use the perimeter to find the missing side.
If we denote the lengths of the known sides as \( S_1, S_2, \ldots, S_n \), and the missing side as \( S_{missing} \), the equation for the perimeter can be expressed as:
\[ P = S_1 + S_2 + \ldots + S_n + S_{missing} \]
Rearranging this gives us:
\[ S_{missing} = P - (S_1 + S_2 + \ldots + S_n) \]
If you know the values for \( S_1, S_2, \ldots, S_n \), you can plug them into this equation along with the perimeter to find \( S_{missing} \).
Since you only provided the perimeter and did not mention the lengths of the other sides of the shape, please provide further information about the known sides so that I can assist you in determining the length of the missing side.