To find the length of the missing side of a shape given the perimeter, we need to take into account the formula for perimeter, which is the sum of all the sides.
If the perimeter of the shape is given as \( 21x^2 - 5x + 4 \), we need to determine the length of the missing side based on the other sides of the shape that you also have (though you haven't provided them).
Since we are not given specific lengths for the other sides in this case, we can formulate the expression for the missing side. Assuming the total of the other sides is represented by \( S \), the expression for the missing side \( M \) can be given by:
\[ M = \text{Perimeter} - S \]
Without the specific lengths for the sides, we cannot compute \( M \). However, if you provide the sum of the known sides, I can help compute the length of the missing side.
If you assume the missing side is one of the four provided options, we can check each option by setting \( M = \text{Perimeter} - S \) equal to each given expression and solving for \( S \).
Please provide more information about the sides of the shape so we can proceed with calculating the missing side.