Quantum Field Theory: 3-4 Equation Steps Explained

I understand how do 3 no. equation come from 1 & 2 no. equation. But I am struggling to understand how do 4 no. equation come from 3 no. equation. Will anyone do the steps between 3 no. equation and 4 no. equation, please ?

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Quantum Field Theory: 3-4 Equation Steps Explained

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