a)
[tex]\it f(-1)+f(2)=-3\cdot(-1)+5+(-3)\cdot2+5=4+5-6+5=8[/tex]
b)
[tex]\it A(3,\ y_A)\in Gf \Rightarrow y_A=f(3)=-3\cdot3+5=-4 \Rightarrow A(3,\ -4)\\ \\ B(x_B,\ 5)\in Gf \Rightarrow f(x_B)=5 \Rightarrow -3x_B+5=5|_{-5} \Rightarrow x_B=0 \Rightarrow B(0,\ 5)\\ \\ \\ AB^2=(x_B-x_A)^2+(y_B-y_A)^2=(0-3)^2+(5+4)^2=9+81=90\\ \\ AB=\sqrt{90}=\sqrt{9\cdot10}=3\sqrt{10}[/tex]
