Need dx coordinate of particles new velocity after applying v2 to particle o1, how do I do this?

public void testD()

{

// p1 = (2,3) // v1 = (-3,0)

Particle o1 = new Particle(p1,v1,10,Color.BLUE);

// v2 = <-6,8>;

o1.applyForce(v2); // accelerate the particle

// Read the assignment carefully if you have problems with this: assertEquals(2.0,o1.getPosition().x());

assertEquals(3.0,o1.getPosition().y());

// Recall that mass of particle is 10 units

assertEquals(???,o1.getVelocity().dx());