f(a)(x) = a*x mod 256  for all x in Z[256]

How do I plot just 10 points of the equation?

I got ..

with(plots):
a := x+2*y = 3;
implicitplot(a, x = -5 .. 5, y = -5 .. 5, style = point);
 

but I only need 10 points. How do I do that? Thank you