[tex]\it c)\ ab-b-a=2|_{+1} \Rightarrow ab-b-a+1=3 \Rightarrow b(a-1)-(a-1)=3 \Rightarrow \\ \\ \Rightarrow (a-1)(b-1)=1\cdot3=3\cdot1 \Rightarrow (a,\ b)\in\{(2,\ 4);\ (4,\ 2)\}\\ \\ \\ b)\ ab-2a-b=12|_{+2} \Rightarrow ab-2a-b+2=14 \Rightarrow b(a-1)-2(a-1)=14 \Rightarrow \\ \\ \Rightarrow (a-1)(b-2)=14 =1\cdot14=2\cdot7=7\cdot2=14\cdot1 \Rightarrow \\ \\ \Rightarrow (a,b)\in\{(2,\ 16);\ (3,\ 9);\ (8,\ 4);\ (15,\ 3)\}[/tex]
