On a coordinate plane, any movement up and down is on the y-axis. So if your parallelogram is moving down 2.5 units, then the y-values of each vertice will go down by 2.5:
A(-2, -8), B(-4, -8), C(-3, -4.5), D(-1, -4.5)
A(-2, -8), B(-4, -8), C(-3, -4.5), D(-1, -4.5)