A: "Things that are equal to the same are equal to each other"
B: "The two sides of this triangle are things that are equal to the same"
C: "If A and B are true, Z must be true"
D: "If A and B and C are true, Z must be true"
E: "If A and B and C and D are true, Z must be true"
Therefore, Z: "The two sides of this triangle are equal to each other"