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You must use the formula that calculates the total energy of an object, which includes both potential and kinetic energy:
$$E_m = E_p + E_k$$
Legend:
• Em: total mechanical energy
• Ep: potential energy
• Ek: kinetic energy

$$E_p = mgh$$
Legend:
• m: mass (kg)
• g: gravitational constant (=9.81)
• h: height (m)

$$E_k = \frac{1}{2}mv^2$$
Legend:
• v: speed (m/s)

This webpage of the Alloprof website explains potential and kinetic energy:
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