а) -7c+3d+8c-5d = (-7c+8c) + (3d-5d) = c - 2d

б) -9+2m-4-m+8 = (-m+2m) + (-9-4+8) = m - 5

в) 3/5k-2/7n-3k-5/7n+0.4k = (3/5 - 3)k + (-2/7 - 5/7)n + 0.4k = (-12/5)k + (-7/7)n + 0.4k = (-12/5)k - n + 0.4k