Rui is a professional deep water free diver. His altitude (in meters relative to sea level), x xx seconds after diving, is modeled by: d ( x ) = 1 2 x 2 − 10 x d(x)= 2 1 x 2 −10xd, left parenthesis, x, right parenthesis, equals, start fraction, 1, divided by, 2, end fraction, x, squared, minus, 10, x How many seconds after diving will Rui reach his lowest altitude?