That is a vague question since we do not know what chapter you are studying in class. But there are two ways to multiply vectors, you can use the dot product or cross product. If the result of the dot product is zero, than both vectors are perpendicular to each other. If the result of the cross product is zero, than both vectors are parallel to each other.
Answer: Either dot product or cross product. But you need to see what chapter you are on.