Answer
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.
