3.
[tex]a = 3m+3n-k\\b = -m+2n-3k\\\\Obw = 2a + 2b = 2(a+b)\\\\Obw = 2(3m+3n-k-m+2n-3k) = 2(2m+5n-4k) = 4m+10n-8k[/tex]
4.
[tex]a) \ (3x-2y)-(-8x-5y) = 3x-2y +8x+5y = 11x+3y\\\\b) \ 8-7m+(-3r)+7m-3+9r-5 = 8-7m-3r+7m-3+9r-5 = 6r\\\\c) \ -12a+6+4a-2+3a = -5a + 4\\\\d) \ -2a(-a+b-1)-(2a-4ab+9a) = 2a^{2}-2ab + 2a -2a+4ab-9a = 2a^{2}+2ab-9a[/tex]