Die korrekte Antwort lautet:
8 - 21 = -13

$(21-\left(20+31\right))·10$

= $\left(21-20-31\right)·10$

= $-30·10$

= $-300$

$2\cdot {\left(-1\right)}^2-4^2$

= $2\cdot 1-16$

= $2-16$

= $-14$