Answer: A) z = 12
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Work Shown:
RS = RT
2z-15 = 9
2z = 9+15
2z = 24
z = 24/2
z = 12
The reason why RS is equal to RT in the first equation is because we have an isosceles triangle. Recall that any isosceles triangle has exactly two sides the same length, and the opposite base angles are congruent to one another. The congruent angles are indicated with the single arcs.
In short: the base angles S and T are congruent, so the opposite sides RS and RT are the same length. This leads to RS = RT.