Applicare la formula: $mx=nx$$\to m=n$, dove $x=y$, $m=4\sec\left(7y\right)$ e $n=-21$

Applicare la formula: $ax=b$$\to x=\frac{b}{a}$, dove $a=4$, $b=-21$ e $x=\sec\left(7y\right)$

Applicare la formula: $a=b$$\to inverse\left(a,a\right)=inverse\left(a,b\right)$, dove $a=\sec\left(7y\right)$ e $b=-\frac{21}{4}$

Applicare la formula: $\mathrm{arcsec}\left(\sec\left(\theta \right)\right)$$=\theta $, dove $x=7y$

Applicare la formula: $ax=b$$\to x=\frac{b}{a}$, dove $a=7$, $b=\mathrm{arcsec}\left(-\frac{21}{4}\right)$ e $x=y$