Verified Answer :

konrad509

Because:
\((a-b)^2=a^2+2ab+b^2\)

\((x-y)^2=x^2-2xy+y^2\\---------------------------\\\\4-12r+9r^2=2^2-2\cdot2\cdot3r+(3r)^2=(2-3r)^2\)

\(other\ method:\\\\4-12r+9r^2=4-6r-6r+9r^2=2(2-3r)-3r(2-3r)\\\\=(2-3r)(2-3r)=(2-3r)^2\)

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