$x^2-\left\{-7xy+\left[-y^2+\left(-x^2+3xy-2y^2\right)\right]\right\}$

$\lim_{x\to4}\left(\frac{\sin\left(\pi x\right)}{x-4}\right)$

$\lim_{x\to\infty}\left(\frac{e^x-x^e}{x-e\cdot\ln x}\right)$

$\frac{dy}{dx}=9xy-9xy^3$

$-2ab\cdot\left(a^{2}b-3ab^{3}\right)$

$0x^{2}+15x+36$

$-6\left(4\right)\left(-9\right)$