Seja $x$ $=$ $\Large{\frac{y-2}{5}}$, assim: $f(y) = 5\cdot f(\large{\frac{y-2}{5}}) + 2 = ay + b$ . Ademais, como: $f(\large{\frac{y-2}{5}}) = a\cdot (\large{\frac{y-2}{5}}) + b$, então $5\cdot f(\large{\frac{y-2}{5}}) + 2$ $=$ $a\cdot (y-2) + 5b + 2$ Logo: $ay-2a + 5b + 2$ $=$ $ay + b$ $\implies$ $4b = 2a-2$ $\implies$ $\boxed{a = 2b+1}$