How to interpolate point?

As we know, to make a contour map interpolation is one thing you shoul have known. The question is, how to make interpolation in 3D coodinates data? Usually, we interpolate points only in 2D coordinates data. It is so simple if we know the procedures.
Here, i give the example of the case. Shown in picture

How to determine the Z coordinate of point a?
Ok, the steps are:

1. Determine the X,Y coordinate of point a'
2. Determine the Z coordinate of point a'
3. Finally, you can determine the Z coordinate of point a'

What is a'? It was a point to help us determine the Z coordinate of point a' . Actually, it was so easy and interesting!

And that was my answer.
Please, correct me if i'm wrong.

Maundri Prihanggo from watershed hydrography class