The proof is in the following form: Column 1 of proof; Column 2 of proof. Thus, we have: 1. $ MN \parallel ST$; Given 2. $RN = NT$; lines parallel to one side of a triangle always proportionally divides the other side of the triangle. 3. MN is the midpoint of RT; the definition of a midpoint is the line that cuts the other line into two smaller lines of equal size.