Screenshot below will hold the question.
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The equation the completes the proof in step 6 is m<w + m<x = m<x + m<z
In step 4 of the proof, we have the following equation
m<w + m<x = m<y + m<z
In step 5, we have
m<x = m<y
When m<x = m<y is substituted to the equation in step 4, we have:
m<w + m<x = m<x + m<z
Hence, the equation the completes the proof in step 6 is m<w + m<x = m<x + m<z
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