Lösung 2.1:1e

If we use the rule for squaring \displaystyle (a-b)^2 = a^2-2ab+b^2 with \displaystyle a=x and \displaystyle b=7, we obtain directly that

\displaystyle (x-7)^2=x^2-2 \cdot x \cdot 7 + 7^2 = x^2-14x+49.

An alternative is to write the square as \displaystyle (x-7)\cdot (x-7) and then multiply the brackets in two steps

\displaystyle \qquad \begin{align} (x-7)\cdot (x-7) &= (x-7)\cdot x - (x-7)\cdot 7 \\ &= x\cdot x-7 \cdot x -(x\cdot 7 - 7\cdot 7) \\ &= x^2 -7x-(7x-49)\\ & \stackrel{*}= x^2-7x-7x+49 \\ &= x^2-(7+7)x+49\\ &= x^2-14x+49 \end{align}

In the line that has been marked with an asterisk, we have removed the bracket and at the same time changed signs on all terms inside the bracket.