To solve for the total length of \( BD \) based on the given segments \( AE \) and \( ED \):
- We have \( AE = 3x \) and \( ED = -4x + 21 \).
- The total length of \( BD \) can be expressed as the sum of \( AE \) and \( ED \): \[ BD = AE + ED = 3x + (-4x + 21) = 3x - 4x + 21 = -x + 21 \]
Next, we need to find \( x \) to determine the total length of \( BD \). However, since the question does not provide a specific value for \( x \), we look to see if there is a pattern evident or specific values provided in the choices given.
Assuming that the problem needs us to analyze \( -x + 21 \):
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We can set the possible values:
- If \( x = 3 \): \[ BD = -3 + 21 = 18 \]
- If \( x = 6 \): \[ BD = -6 + 21 = 15 \]
- If \( x = 0 \): \[ BD = -0 + 21 = 21 \]
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Among the answers, the closest consistent value derived from the expression \( -x + 21 \) when \( x = 3 \) gives us \( 18 \).
Therefore, based on the potential values and given the options, the total length of \( BD \) is:
18.