a/ \((6n+1)(n+5)-(3n+5)(2n-1)\\=6n^2+31n+5-(6n^2+7n-5)\\=6n^2+31n+5-6n^2-7n+5\\=24n+10\\=2(12n+5)\\→2(12n+5)\vdots 2\\→(6n+1)(n+5)-(2n+5)(2n-1)\vdots 2\)
b/ \( (n-1)(3-2n)-n(n+5)\\=-2n^2+5n-3-n^2-5n\\=-3n^2-3\\=3(-n^2-1)\\→3(-n^2-1)\vdots 3\\→(n-1)(3-2n)-n(n+5)\vdots 3\)