$12a-\left(-3a\right)$

$3c-9c+\left(-3c\right)+4c+5c$

$-6k+15k-13k$

$-\left(-3a\right)+5a-4a+\left(-a\right)$

$\left(-2d+e\right)-\left(5d+4e\right)-2d+3e$

$12x-\left(12x+3y\right)+\left(-3y\right)-\left(3x-y\right)$

$-6m-\left(4-6m\right)+3m+\left(4-3m\right)$

$a-b-c-d-\left(a-b-c-d\right)+\left(a-b-c-d\right)$

$7m-5n-\left[5m-\left(3n-m\right)-\left(2m+n\right)-5n\right]$

$\left[7a+5b-\left(3a+b\right)\right]-\left\{\left[3b-\left(2a-b\right)\right]-5a\right\}$