Write the equation of the function graphed?

There can be an infinite of a priori answers. Are you looking for a particular form of f(x) ?
Here is one that seems to fit. Hint: Consider f(x) = -1 * (x^(1/3)).
Then translate it to h=1.25 & k = 2.
Now, make it fit with a parameter k: F(x) = -1 * k(x^(1/3)). For this, use the other given point (1,3) once translated to (h,k).