Invert an integral equation to find this unknown

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Replusz
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TL;DR
I don't quite understand the steps taken to get to the second line (3.71->3.72):
(http://www.damtp.cam.ac.uk/user/tong/qft/qft.pdf)
Two displayed equations from a printed page: integral of d^3r U(r) e^(-i(p-p')·r) equals -λ² over ((p-p')² + m²), labelled (3.71); and U(r) = -λ² times integral of d^3p/(2π)^3 times e^(ip·r) over (p² + m²), labelled (3.72), with page number 68 between them.
 
Apart from the insane habit in physics to write the measure in front of the integrand (:wink:) and other conventions related to factors of ##\pi##, this follows from taking inverse Fourier transforms on both sides of (3.71).
 
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Thank you for your quick and perfectly clear reply! :)
 
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