4.
a)
[tex]\it A\cap \mathbb{N}=\{0,\ 3\}\\ \\ (x,\ 2x)\in A\times B\\ \\ x=0 \Rightarrow 2x=2\cdot0=0\not \in B\\ \\ x=3 \Rightarrow 2x=2\cdot3=6\in B\\ \\ Deci,\ \ x=3[/tex]
b)
[tex]\it (y,\ y+2)\in A\times B \Rightarrow y\in A,\ \ y+2\in B\\ \\ y=-1 \Rightarrow y+2=-1+2=1\in B\\ \\ y=0 \Rightarrow y+2=0+2=2 \not\in B\\ \\ y=3 \Rightarrow y+2=3+2=5 \not\in B\\ \\ Deci,\ \ y=-1[/tex]
