Diabolical Engine I

A physics student, irritated by the personal mannerisms of their physics professor, decides to build an infernal device to rid the world of him altogether. The student plans to push a large, massive object (the statue of Washington Duke, actually) from a tall building of height $H$ by means of a powerful spring (with spring constant $k$) stolen from the suspension of a jacked-up Camero. However, the student (being a careful sociopath) wants to make sure that the mass $M$ will make it over the roses to the path a distance $D$ from the base of the building.

Unfortunately, the student isn't very good at physics and comes to you for help. Since they don't want to tell you which building or which path or spring or mass they want to use (you might be able to testify against them!) they want you to find a general formula for how far to compress the spring.

a) Help them out. Find $\Delta x = x - x_0$ in terms of $H$, $M$, $D$, $k$ and $g$ (the gravitational constant) that will drop $M$ on RGB assuming no friction on the roof. That way I'm still pretty safe.

b) Explicitly show that your answer to a) has the right units. If you are clueless in a) try to find SOME combination of these letters that has the right units and varies the way you expect the answer to (more height $H$ means smaller $\Delta x$, for example, so it probably belongs on the bottom).