Question

B={X∈N/7supra x-2 ∈N} 

Answer 1

7/x-2
x-2 apartine divizorilor lui 7 {1,7}
x-2=1            x-2=7
x=1+2           x=7+2
x=3              x=9
x apartine {3;9}

Answer 2

[tex]B=\{x\in N| \frac{7}{x-2}\in N \} \\ \frac{7}{x-2} \in N \\ x-2|7 \\ x-2 \in\{1,7\}|+2 \\ x \in \{3,9\}[/tex]