Answer:
C
Explanation:
Since triangles TUS and QRS are similar, if TUS is isosceles then so is QRS. This means that all corresponding proportions are at a constant ratio. So if TS=US, QT=RU and therefore a=7. Hope this helps!
a =7
The triangles are similar so we can use ratios
10 10
------ = --------
10+a 10+7
Since the numerators are the same, the denominators have to be the same
10+a = 10+7
a = 7
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