Answer
The proof is in the following form:
Column 1 of proof; Column 2 of proof.
Thus, we have:
1. $ XY \parallel RT$ and $YZ \parallel RS$; Given
2. $\frac{YT}{SY} = \frac{RX}{XS}$ and $\frac{ZT}{RZ}= \frac{YT}{SY}$; lines parallel to one side of a triangle always proportionally divides the other side of the triangle.
3. $ \frac{RX}{XS} = \frac{ZT}{RZ}$; transitive property (two things equal to the same thing are equal to each other.)
