For the function $\displaystyle y=\sqrt{x}$ determine the equation of the tangent at the point on the graph where $\displaystyle x=9$
the gradient of that equation at x = 9 is just the derivitive
gradient (m) = dy/dx = 0.5x^-0.5
a tangent will have the form of y = mx + c
and from the original equation can see that when x = 9, y = 3;
using all these values you should be able to determing the equation
For further help see
Tangents and Normals