I suppose that you should have given some pairs to choose from.
Anyway, I can tell you how to do it. Given a pair [tex](a,b)[/tex], you have to substitute [tex]a[/tex] and [tex]b[/tex] to [tex]x[/tex] and [tex]y[/tex] respectively, and see if you get something true.
For example, the pair [tex](1,7)[/tex] satisfies the function, because the substitution yields
[tex]7=4\cdot 1+3[/tex]
which is true. On the other hand, the pair [tex](19,3)[/tex] doesn't satisfy the function, because
[tex]3=4\cdot 19+3[/tex]
is clearly false.
