\[\boxed{\text{6.278.}}\]
\[\textbf{а)}\ 2,3x + 4,7x - 4,3x =\]
\[= 2,3x + (4,7x - 4,3x) =\]
\[= 2,3x + 0,4x = 2,7x\ \]
\[\textbf{б)}\ 3,6y - 1,4y + 1,8y =\]
\[= 3,6y + (1,8y - 1,4y) =\]
\[= 3,6y + 0,4y = 4y\ \]
\[\textbf{в)}\ 0,85m - 0,37m - 0,43m =\]
\[= 0,85m - (0,37m + 0,43m) =\]
\[= 0,85m - 0,8m = 0,05m\ \]
\[\textbf{г)}\ 8,3k - 4,3k + 1,6k =\]
\[= (8,3k - 4,3k) + 1,6k =\]
\[= 4k + 1,6k = 5,6k\ \ \]