$11+\left(-9-4\right)+15$

= $11-13+15$

= $-2+15$

= $13$

$90:\left(-3-\left(-12\right)\right):\left(3+7-20\right)$

= $90:\left(-3+12\right):\left(10-20\right)$

= $90:9:\left(-10\right)$

= $10:\left(-10\right)$

= $-1$

$11-\left(-6\right)+\left(-24-\left(-6\right)\right)$

= $11+6+\left(-24+6\right)$

= $17-18$

= $-1$