a: =>-12-x+3=5+7=12
=>-x-9=12
=>-x=21
hay x=-21
b: \(\Leftrightarrow14-x-10=9+15=24\)
=>4-x=24
=>x=-20
c: \(\Leftrightarrow12+17-3=x-5\)
=>x-5=26
hay x=31
d: \(\Leftrightarrow21-25+x-21=-25+7\)
=>x=7
e: \(\Leftrightarrow12+23-x=23-5+12\)
=>-x=-5
hay x=5