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Explanation:

Since it is an equilateral Triangle their sides are the same.

Thus, 4(x - 1) = x + 8 = 2x + 4

But we can take 2 sides to work with:
To solve for x:

4(x - 1) = x + 8
4x - 4 = x + 8
4x - x = 8 + 4
3x = 12
x = 12/3
x = 4

Therefore, x = 4 and the equation involving x is 4(x - 1) = x + 8