$5·\left(-20\right):\left(-1+11\right)$

= $-100:10$

= $-10$

$(-3·\left(5-20\right)):\left(1-18-\left(-12\right)\right)$

= $\left(-3\left(5-20\right)\right):\left(-17+12\right)$

= $(-3·\left(-15\right)):\left(-5\right)$

= $45:\left(-5\right)$

= $-9$

$21:3-18:6·\left(7-11\right)$

= $7-3·\left(-4\right)$

= $7+12$

= $19$