$3\left(x+4\right)-6x=8-3\left(x-5\right)$

$12x+8=20$

$7a^{3}+14a^{2}+21$

$\left(\sqrt{5}\right)\left(-\sqrt{5}\right)$

$\frac{1}{\left(\cos\left(x\right)+1\right)}+\frac{1}{\left(\cos\left(x\right)-1\right)}=2\cdot\csc\left(x\right)\cdot\cot\left(x\right)$

$\lim\:_{x\to\:-3}\left(\frac{x^4-81}{x+3}\right)$

$\left(-4\right)\left(-21\right)$