$$\boxed{y = f(x) = 1 - 3x}$$
a) $+)\quad f(1) = 1 - 3.1 = -2$
$+)\quad f(0) = 1 - 3.0 = 1$
$+)\quad f\left(\dfrac13\right) = 1 - 3\cdot\dfrac13 = 0$
$+)\quad f(2) = 1 - 3.2 = -5$
b) $+)\quad y = 3$
$\Leftrightarrow 1 - 3x = 3$
$\Leftrightarrow 3x = -2$
$\Leftrightarrow x = -\dfrac23$
$+)\quad y = -5$
$\Leftrightarrow x = 2$ (Dựa vào câu a)