$\left(xy-4\right)^2\cdot\left(xy-4\right)$

$12a-8a+10a+3a-18a+5a$

$\left[\left(+4\right)-\left(-7\right)\right]-\left(-2\right)$

$x^4+y^4\:=\:81.\:$

$\frac{3x^2+x+6}{-2x+6}$

$\left(\left(-3\right)^9.\left(-2\right)^8\right):\left(\left(-3\right)^7.\left(-2\right)^5\right)$

$\left(s^7t^0u^{89}\right)\left(st^0u^0\right)\left(s^{-6}t^5u^{-85}\right)$