In the language of modular arithmetic, we're given
[tex]x-3\equiv0\pmod7\implies x\equiv3\pmod7[/tex]
[tex]y+3\equiv0\pmod7\implies y\equiv-3\equiv4\pmod7[/tex]
Then x = 7a + 3 and y = 7b + 4 for integers a and b.
Substitute these into the quadratic expression and simplify:
[tex]x^2+xy+y^2+n\equiv0\pmod7[/tex]
[tex](7a+3)^2+(7a+3)(7b+4)+(7b+4)^2+n\equiv0\pmod7[/tex]
[tex]49a^2+42a+9+49ab+28a+21b+12+49b^2+56b+16+n \equiv 0\pmod7[/tex]
[tex]37+n\equiv 0\pmod7[/tex]
[tex]n\equiv-2\equiv5\pmod7[/tex]
which means the smallest positive integer n we are looking for is 5.
