Answer:
4f-7
Step-by-step explanation:
You want the perimeter of a triangle whose side lengths are f, 2f, and f-7.
Perimeter
The perimeter is the sum of the side lengths:
P = f +2f +(f -7)
P = 4f -7
The perimeter of the triangle is 4f-7.
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Additional comment
No such triangle can exist. The sum of the two shortest sides is f+(f-7) = 2f-7, which is not greater than the longest side, 2f. The triangle inequality is not satisfied.
