We know that the sum of the lengths of the sides of a polygon is the perimeter of the polygon, by definition.
Thus, in our case of four sided polygon with the side lengths as a,b,c and d and the perimeter as P, we can write the equation of the perimeter to be as:
![P=a+b+c+d](https://tex.z-dn.net/?f=%20P%3Da%2Bb%2Bc%2Bd%20)
Thus, if we have to find the side length, d, given the perimeter, P , and other three sides a,b and c, then we will have to isolate the d in the above equation by subtracting a+b+c from both sides. We will get:
![P-a-b-c=d](https://tex.z-dn.net/?f=%20P-a-b-c%3Dd%20)
Thus, the the equation that can be used to find the side length, d is:
![d=P-a-b-c](https://tex.z-dn.net/?f=%20d%3DP-a-b-c%20)
From the given list Option B is the correct option.