Applicare la formula: $\frac{a}{b}\frac{c}{f}$$=\frac{ac}{bf}$, dove $a=7$, $b=3m$, $c=9m$, $a/b=\frac{7}{3m}$, $f=21$, $c/f=\frac{9m}{21}$ e $a/bc/f=\frac{7}{3m}\frac{9m}{21}$

Applicare la formula: $ab$$=ab$, dove $ab=3\cdot 21m$, $a=3$ e $b=21$

Applicare la formula: $ab$$=ab$, dove $ab=7\cdot 9m$, $a=7$ e $b=9$

Applicare la formula: $\frac{a}{a}$$=1$, dove $a=63m$ e $a/a=\frac{63m}{63m}$

Applicare la formula: $\frac{a}{a}$$=1$, dove $a=63m$ e $a/a=\frac{63m}{63m}$