Is the equation for slope "M= y1-y2/x1-x2" remotely valid

The equation you have given for the gradient will work out exactly the same as Heureka's so it is perfectly valid.

Is the equation for slope "M= y1-y2/x1-x2" remotely valid ?

$$\boxed{ m= \dfrac{y_2-y_1}{x_2-x_1} }$$