Given two points in a Cartesian coordinate system, find the x-value, where polynomial of 1. order (y = m*x+c) is equal to a given value c.
e.g.: we have the points P1(1,2) and P2(3,6) and we want wo know at which x-value y is equal to c = 8; The input is: x = [1 3], y = [2,6], c = 8.
The answer is 4.
Consider the possibility that there isn't any solution (return NaN) of that there is an infinity amount of points (return Inf). Additionally consider that the point could represent a vertial line.