Answer: Parallelogram
You could use the parallelogram rule to add the vectors, or you could use the tip-to-tail method (your textbook might call it the "head to tail method" but it's the same idea).
An example of the parallelogram method is shown below with adding the vector u = (-4,4) in red to the vector v = (8,2) in blue to get the vector w = (4,6) in green. The green resultant vector is one of the diagonals of the parallelogram
Side note: The other diagonal is either u-v or v-u depending on your reference point.
