To find the total length of segment \( BD \) in the rectangle \( BCDA \) with given lengths \( AE = 3x \) and \( ED = -4x + 21 \), we can use the property of segments intersecting within a quadrilateral.
From the diagram and description, we have:
- Segment \( AE \) and segment \( ED \) are parts of the diagonal \( AD \), and point \( E \) is where segments \( AC \) and \( BD \) intersect.
Using the property of segments in a quadrilateral, we can express the total length of diagonal \( BD \) as the sum of the lengths of segments \( AE \) and \( ED \).
Thus, we can establish the equation for \( BD \) as follows:
\[ BD = AE + ED \]
Substituting the expressions for \( AE \) and \( ED \):
\[ BD = 3x + (-4x + 21) \]
Now, simplifying the equation:
\[ BD = 3x - 4x + 21 \]
\[ BD = -x + 21 \]
So the total length of \( BD \) can be expressed as:
\[ BD = -x + 21 \]
To determine the exact value of \( BD \), you will need to know the value of \( x \). If \( x \) is provided or can be calculated from other information, substitute it into the equation \( BD = -x + 21 \) to find the length.