-4x^2-x=f(3)

If you mean f(x) = -4x² - x and you want to know f(3) then DarkBlaze347 has supplied the right answer.

.

$${{\mathtt{3}}}^{{\mathtt{2}}} = {\mathtt{9}}$$

$${\mathtt{9}}{\mathtt{\,\times\,}}-{\mathtt{4}} = -{\mathtt{36}}$$

$${\mathtt{\,-\,}}{\mathtt{36}}{\mathtt{\,-\,}}{\mathtt{3}} = -{\mathtt{39}}$$

f(3)=-39.