\[\boxed{\text{913.}\text{\ }еуроки - ответы\ на\ пятёрку}\]
\[\textbf{а)}\ (2x + y)^{2} - (x - 2y)^{2} = (2x + y - x + 2y)(2x + y + x - 2y) =\]
\[= (x + 3y)(3x - y)\]
\[\textbf{б)}\ (a + b)^{2} - (b + c)^{2} = (a + b - b - c)(a + b + b + c) =\]
\[= (a - c)(a + 2b + c)\]
\[\textbf{в)}\ (m + n)^{2} - (m - n)^{2} = (m + n - m + n)(m + n + m - n) =\]
\[= 2n \cdot 2m = 4nm\]
\[\textbf{г)}\ (4c - x)^{2} - (2c + 3x)^{2} = (4c - x - 2c - 3x)(4c - x + 2c + 3x) =\]
\[= (2c - 4x)(6c + 2x) = 4 \cdot (c - 2x)(3c + x)\ \]