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

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

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