$\begin{array}{l}a-x+x-a+x-a+2x\\ =a-a-a-x+x+x+2x\\ =-a+3x\end{array}$

$\begin{array}{l}2xy-y+a+2y+y^2\\ =y^2-y+2y+2xy+a\\ =y^2+y+2xy+a\end{array}$

$\begin{array}{l}-14a-(-7+2a)\\ =-14a+7-2a\\ =-16a+7\end{array}$

$\begin{array}{l}-14a-(7-2a)\\ =-14a-7+2a\\ =-12a-7\end{array}$

$\begin{array}{l}2x(3x+1)\\ =6x^2+2x\end{array}$

$\begin{array}{l}2x(3x\cdot 1)\\ =6x^2\end{array}$

$\begin{array}{l}{x}^3\cdot x^7\\ =x^{3+7}\\ =x^{10}\end{array}$

$\begin{array}{l}(-1)\cdot {\left(x^3\right)}^2\\ =(-1)\cdot x^{3\cdot 2}\\ =(-1)\cdot x^6\\ =-x^6\end{array}$

$\begin{array}{l}{(-x^3)}^2\\ =((-1)\cdot x^3{)}^2\\ =(-1{)}^2\cdot (x^3{)}^2\\ =1\cdot x^{3\cdot 2}\\ =x^6\end{array}$

$\begin{array}{l}(3x-1)(5x^2-2x)\\ =15x^3-6x^2-5x^2+2x\\ =15x^3-11x^2+2x\end{array}$

$\begin{array}{l}(uv-w^2)(uw+v^2)\\ =u^{2}vw+u{v}^3-u{w}^3-w^2{v}^2\end{array}$

$\begin{array}{l}(x+1)(x-2)(x+3)\\ =(x^2-2x+x-2)(x+3)\\ =(x^2-x-2)(x+3)\\ =x^3-x^2-2x+3x^2-3x-6\\ =x^3-x^2+3x^2-2x-3x-6\\ =x^3+2x^2-5x-6\end{array}$

$\begin{array}{l}7x^2-[x-x(3x+1)]\\ =7x^2-(x-3x^2-x)\\ =7x^2-(-3x^2)\\ =7x^2+3x^2\\ =10x^2\end{array}$

$\begin{array}{l}{(3a+b)}^2\\ ={\left(3a\right)}^2+2\cdot 3a\cdot b+b^2\\ =9a^2+6ab+b^2\end{array}$

$\begin{array}{l}{(\frac{2}{3}-a)}^2\\ =\frac{4}{9}-\frac{4}{3}a+a^2\end{array}$

$\begin{array}{l}{\left(\frac{2}{3}a\right)}^2\\ =\frac{4}{9}{a}^2\end{array}$

$\begin{array}{l}x(x-1)(x+3)-x^2(1+x)\\ =(x^2-x)(x+3)-x^2-x^3\\ =x^3+3x^2-x^2-3x-x^2-x^3\\ =x^3-x^3+3x^2-x^2-x^2-3x\\ =x^2-3x\end{array}$

$\begin{array}{l}10{(x-\frac{2}{5})}^3-\mathrm{0,8}(6x-\mathrm{0,8})\\ =10(x-\frac{2}{5})(x^2-\frac{4}{5}x+\frac{4}{25})-\mathrm{0,8}(6x-\mathrm{0,8})\\ =(10x-4)(x^2-\frac{4}{5}x+\frac{4}{25})-\mathrm{4,8}x+\mathrm{0,64}\\ =10x^3-8x^2+\frac{8}{5}x-4x^2+\frac{16}{5}x-\frac{16}{25}-\mathrm{4,8}x+\mathrm{0,64}\\ =10x^3-8x^2-4x^2+\frac{8}{5}x+\frac{16}{5}x-\mathrm{4,8}x-\frac{16}{25}+\mathrm{0,64}\\ =10x^3-12x^2\end{array}$