To find the length of the missing side, we need to know the perimeter of the shape, which is given as \(21x^2 - 5x + 4\).
Let's assume that the shape has four sides, and we have three of them with their lengths represented as \(a\), \(b\), and \(c\). The missing side we'll call \(d\). The formula for the perimeter is:
\[ P = a + b + c + d \]
In this case, we have:
\[ 21x^2 - 5x + 4 = a + b + c + d \]
To find \(d\), we would need the total lengths of \(a\), \(b\), and \(c\). Since we don't have those values, we generally take \(a + b + c\) to be some expression that we will represent as \(S\):
\[ d = (21x^2 - 5x + 4) - S \]
However, without further information about \(a\), \(b\), and \(c\), we cannot determine \(d\) exactly.
Please provide the values of the other three sides, or at least their algebraic expressions so we can proceed to calculate the missing side.