find an equation of the line that is tangent to the graph of

$\displaystyle f(x) = \frac{1}{\sqrt{x}}$

and parallel to the line of $\displaystyle x + 2y - 6 = 0$

I know that

$\displaystyle \frac {d}{dx} \left(\frac{1}{\sqrt{x}}\right) = -\frac{1}{2x^{3/2}}$

and that the parallel line can be rewritten as

$\displaystyle y = -\frac{1}{2}x+3$

but not sure how you use this to get the equation of the line that is tangent to the graph.