"Line crosses points M and N:
M(2;6), N(-2;-6), find the equation for it"
The only formula I found was
It works, but this is grade 9 (in our country) exercise, I think there should be a simpler way. Is there?
In essence, any method you use is going to compute what you have there. The only equation for a line that I ever use is y = mx + b. For the case of two points on the line, I just plug both points into the y = mx + b equation to obtain two simultaneous equations. I solve that using any of the many valid methods, and then I'm done.