$\frac{2}{x^2}-<!--\; -\; -->\frac{3}{x^3}=\frac{2x-<!--\; -\; -->3}{x^3}$

$\frac{7}{m^4}-<!--\; -\; -->\frac{3a}{m^2}=\frac{7-<!--\; -\; -->3a{m}^2}{m^4}$

$\frac{1}{a^5{b}^3}+\frac{1}{a{b}^7}=\frac{b^4+a^4}{a^5{b}^7}$

$\frac{4}{x^4{b}^3}-<!--\; -\; -->\frac{3}{x^2{b}^5}=\frac{4b^2-<!--\; -\; -->3x^2}{x^4{b}^5}$

$\frac{3a}{x^7{y}^{5}z}-<!--\; -\; -->\frac{3b}{x{y}^4{z}^5}=\frac{3a{z}^4-<!--\; -\; -->3b{x}^{6}y}{x^7{y}^5{z}^5}$

$\frac{m^{7}n}{a^4{b}^3{c}^9}+\frac{3m{n}^2}{a^3{b}^6{c}^4}=\frac{m^{7}n{b}^3+3m{n}^{2}a{c}^5}{a^4{b}^6{c}^9}=\frac{mn(b^3{m}^6+3a{c}^{5}n)}{a^4{b}^6{c}^9}$