In equilateral triangles STU:

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Explanation:
Equilateral triangles have three equal sides, ie the sides are all the same length.
Let's set sides ST and TU equal to each other and solve for x
7x-10 = 2x+25
7x-2x = 25+10
5x = 35
x = 35/5
x = 7
Then we use this to compute each side
Each side is 39 units long, so this confirms we have the correct answer.