A = (-2a + 3b - 4c) - (-2a - 3b - 4c)
= -2a + 3b - 4c +2a + 3b + 4c
=6b
\(A=\left(-2a+3b-4c\right)-\left(-2a-3b-4c\right)\)
\(A=-2a+3b-4x+2a+3b+4c\)
\(A=\left(2a-2a\right)+\left(4c-4c\right)+3b+3b\)
\(A=3b+3b\)
\(A=\left(-2a+3b-4c\right)-\left(-2a-3b-4c\right)\)
\(\Rightarrow A=-2a+3b-4x+2a+3b+4c\)
\(\Rightarrow A=\left(2a-2a\right)+\left(4c-4c\right)+3b+3b\)
\(\Rightarrow A=3b+3b\)
\(\Rightarrow A=6b\)
\(\text{Vậy }A=6b\)
