$2y-7x+3x=2y-4x$

$\mathrm{0,1}{a}^2\cdot 30a^3=\mathrm{0,1}\cdot 30\cdot a^{2+3}=3\cdot a^5$

${\left({\displaystyle \frac{1}{3}}{x}^3\right)}^2={\displaystyle \frac{1}{3^2}}{x}^{3\cdot 2}={\displaystyle \frac{1}{9}}{x}^6$