How to calculate a variable x that produces non-linear results? Sorry I don't have a mathematical background so It is hard for me to put the right words on it.
Let me explain what I am trying to do with an example.
Essentially I want to solve x so that it satisfies both situation:
$100 \times x = 0$
$50 \times x = 3$
I would like to know how to solve this type of equation as well as to know the name of the mathematical concepts involved here.
 A: To find value of a single unknown variable only one equation is adequate.
An extra equation is termed redundant.
You have given two.
The first  equation gives as a solution $x = 0 $
The second equation gives as a solution $x = 0.6$
Both cannot be true at the same time. In other words the
both cannot be simultaneously true.
A: I'm going to interpret your question a little differently, since you yourself say that your mathematical understanding is a bit limited. What I think you want is not a variable $x$ but rather some kind of function $f$ which for the value 100 gives you 0, and for the value 50 gives you 3. I.e., we have
$$f(100) = 0$$
$$f(50) = 3$$
Now, there are infinitely many different functions that would satisfy the two requirements you have. But if we for example stick to the simplest solution, a linear function, the solution would be
$$f(x) = \frac{-3x}{50} + 6$$
From this you can find a new value, for example which one corresponds to 75, by:
$$f(75) = \frac{-3 \cdot 75}{50} + 6 = 1.5$$
