Equation of a line is given as y = mx + b, where m is the gradient (angle) and b is the y-intercept at x=0. Define a function that given two points through which line passes, returns the gradient of the line. Return null if it is a vertical line (hence, the return type Double) If you don't have the required math knowledge, refer to this page: See this to further understand this: https://www.mathsisfun.com/equation_of_line.html