Step-by-step explanation:
x = BD
[tex] \frac{ab}{ad} = \frac{bc}{de} \\ \frac{10}{10 + bd} = \frac{12}{21} \\ 12 \times (10 + bd) = 21 \times 10 \\ 120 + 12bd = 210 \\ 12bd = 210 - 120 \\ 12bd = 90 \\ bd = \frac{90}{12} \\ bd = 7.5 \\ [/tex]
