Question

Factor by grouping:

Answer 1

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

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

Answer 2

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