sec x - sq rt 2 tan x (the 2 is the only thing under the sq rt and it's being multiplied by tan x)
do you have to use the definition of derivative to do this? if not, it is very easy to look on the formula sheet in your text and see that $\displaystyle \frac d{dx} \sec x = \sec x \tan x$ and $\displaystyle \frac d{dx} \tan x = \sec^2 x$