From Förberedande kurs i matematik 1

Using the distributivity rule, we can multiply the factor \displaystyle 3x into the bracket \displaystyle (x-1) . Each term in \displaystyle (x-1) is then to be multiplied by \displaystyle 3x,

\displaystyle 3x(x-1)=3x\cdot x - 3x \cdot 1 = 3x^2-3x\,\textrm{.}