The third option is the right one. A standard linear equation is represented by y=mx+c where m is the slope and c is a constant. The point (2,-4) represents (x,y). If you substitute the x value (2) in each of the equations:
For the first choice, you get y= 6 so it's wrong
For the second choice, you get y= -2 so it's wrong as well
For the third choice, you get y= -4. This is the right answer
For the last choice, you get y= 4 which is wrong
Hope this helps, let me know if you have any questions