lets set our x's apart first, the 5 can only be multiplied by 1 and 5 to get it so lets put a 5 in one side and a 1 in the other... it should look like (5x )(1x )
now we have a postive in the first term and a negative in the second, this means that there will be a positive and a negative sign... now lets see what can be multiplied by 50 that can add to be 15.. lets try 25 times 2 it gives you 50 but lets check if it gives you 15 in the middle,,lets set it up like this (5x+25)(1x-2) if you multiply these will you get 5x2+15x-50?? yes you will!! so which of these factors are alike to your answer?? well the only one would be (x-2), this means your answer would be letter C
Hope this helps