similar to what wilmer said:
BEF is similar to BCD, so
$\displaystyle \frac{X}{Y} = \frac{100}{320}$
$\displaystyle X = \frac{100y}{320}$
ABD is similar to EFD, so
$\displaystyle \frac{210}{320} = \frac{X}{320-Y}$
$\displaystyle \frac{210 \times(320-Y)}{320} = X$
You have a pair of simulaneous equations to solve.
$\displaystyle \frac{210 \times(320-Y)}{320} = \frac{100y}{320}$
solve for Y (gives about 216.774)