a, 345-[12-42+345]=345-12+42-345=-12+42=30
b, (3965-2378)- (437-1378)-528= 3965-2378-437+1378-528=3965-(2378-1378)-(437+528)
= 3965-1000-965=(3965-965)-1000= 3000-1000=2000
c, 173-(36+173)+[(175-27)-175-(-36+50)]= 173-36-173+ [175-27-175+36-50]
= -36+-27+36-50=-27-50=-77