sqrt(x) +sqrt(y) = 7

Differentiate both sides with respect to x,

1/(2sqrt(x)) +[1/(2sqrt(y)](y') = 0

y' = -[1 / 2sqrt(x)] / [1 / 2sqrt(y)]

y' = -[1 /sqrt(x)] * [sqrt(y) / 1]

y' = -sqrt(y) / sqrt(x)

y' = -sqrt(y/x)

y(16) = 9

So, point (16,9)

y'(16) = -sqrt(9/16) = +,-3/4 ----------------answer.