Inverses in Truncated Polynomial Rings
$$(-10+22x+22x^3-22x^6) - 1 = -11+22x+22x^3-22x^6 \equiv 0 \mod 11.$$

When substracting a constant from a polynomial, you do not subtract it from every term, only from the constant term.

If you need a refresher, see addition and subtraction of polynomials.