Die korrekte Antwort lautet:
-31 - 23 = -54

$(28-\left(21+38\right))·20$

= $\left(28-21-38\right)·20$

= $-31·20$

= $-620$

$2\cdot {\left(-1\right)}^3-5+{\left(-3\right)}^2$

= $2\cdot \left(-1\right)-5+9$

= $-2-5+9$

= $2$