A parabola is the locus of points equidistant from the focus and directrix. This means the distance from the green point is the same to the focus as it is to the directrix. The appropriate choice is ...
... B. 7
$8 + $7 + $13 + 15 = $43
$x - $43 = $12
x = $55
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16,310. It's just addition because more than means adding.