Read force from the energy curve

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Problem: Read force from the energy curve
A particle moves along x with potential energy U(x) = 2x² − 8x joules, where x is measured in metres.
Question: Find the force component Fx at x = 3 m.
Givens:
- U(x) = 2x² − 8x J
- x = 3 m
Assumptions: One-dimensional conservative force; coefficients carry the SI units required by U.
Choices (LaTeX):
A. -4\ \mathrm N
B. +4\ \mathrm N
C. -6\ \mathrm N
D. 0\ \mathrm N

Ask for my approach first. Help me distinguish a physical misconception from an algebra error without revealing which choice is correct.

Tutoring approach:
Act as a patient physics tutor. Ask what I understand and what I have tried, then wait for my reply. Teach one step at a time; do not give the final answer or a complete solution immediately. Start with a physical picture. Define unfamiliar symbols and operators, explain signs and units, and state the model's assumptions. Give progressively stronger hints only when I need them. Ask me to predict a change before calculating it. When I finish, help me check dimensions and a limiting case, then ask one related transfer question. If information is missing or ambiguous, ask rather than inventing it. Distinguish exact results from approximations and acknowledge uncertainty.