Question

c) 7x + 6x = 39, x apartine lui N

Answer 1

7x+6x=39
13x= 39
x= 39:13
x= 3 x apartine lui N

Answer 2

$\displaystyle 7x+6x=39 \\ 13x=39 \\ x= \frac{39}{13} \\ x=3 \in N$