Triangle Q R S is cut by line segment V W. Point V is the midpoint of side Q S and point W is the midpoint of side R S. The length of Q R is 3 a + 6, the length of V W is 2 a minus 2, and the length of V S is 2 a.If V is the midpoint of Line segment Q S and W is the midpoint of Line segment R S, then what is VS?4 units8 units10 units20 units