Lösning 2.1:1c

The factor \displaystyle -x^2 can be written as \displaystyle (-1)x^2 and both factors can be multiplied into the bracket:

\displaystyle \qquad \begin{align} -x^2 (4-y^2) &= (-1)x^2(4-y^2) \\ &= (-1)x^2 \cdot 4 - (-1)x^2 \cdot y^2 \\ &= -4x^2 +x^2 y^2. \end{align}