The quadrilateral is on a graph and I don't know how to find the perimeter of it.
The perimeter is found by adding the lengths of all 4 sides. Since your quadrilateral is on a graph, I assume you know the end points of each side? If the sides are all horizontal or vertical, then the lengths would be simply the difference between the x or y points (x for horizontal sides, y for vertical sides).
However, if your quadrilateral is "tilted" then you will have to create some right trianges and use the Pythagorean Theorum (a^2 + b^2 = c^2) to find the lengths.
I've given a general answer, but hopefully it is enough of a hint to get you going.