Explicación paso a paso:
1) 2-[6-(-5+7)]
2-[6+5-7]
2-6-5+7
=-2
2) 1-(-9+4)-[(-8-1)+5]+9
1+9-4-[-8-1+5]+9
1+9-4+8-1+5+9
=27
3) - {-6+7-[1-8-(4-2)+10]-3}
-{-6+7-[1-8-4+2+10]-3}
-{-6+7-1+8+4-2-10-3}
+6-7+1-8-4+2+10+3
=3
4) 7-{4-[-(-12+3)-6]-7-(-2)}
7-{4-[+12-3-6]-7+2}
7-{4-12+3+6-7+2}
7-4+12-3-6+7-2
=11
