From Förberedande kurs i matematik 1

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

\displaystyle \begin{align} -x^2 (4-y^2) &= (-1)x^2(4-y^2)\\[3pt] &= (-1)x^2 \cdot 4 - (-1)x^2 \cdot y^2\\[3pt] &= -4x^2 +x^2 y^2\,\textrm{.} \end{align}