When you add two forces the resultant force is determine according to the rules of vector addition. It will be a minimum if the two forces are pointing in opposite directions -180 degrees apart - and at a maximum if the two forces are pointing in the same direction, or 0 degrees apart. If the angle between them is in between 0 and 180 then the combined force has a magnitude somewhere in between, . So the minimum possible value is with the 4N force and the 11N force acting in opposite directions, and that magnitude is 11N - 4N = 7N. Hence any value less than 7N is impossible. Similarly any value greater than 11N + 4N = 15N is also impossible. But all values between 7N and 15N are possible (depending on the angle between the two forces).