i have a suface $\displaystyle \sqrt{x}+\sqrt{y}+\sqrt{z}=\sqrt{a}$

find the formula of a plane which is tangent to it in (x0,y0,z0)

i need to find the vector whic is vertical to this surface

the formula of it is $\displaystyle <f_x,f_y,-1>$

but i dont know what is the formula for f(x,y) ,

i have a formula in other form

how to write it in the f(x,y) form

?