You can sum terms that involve the same literal expressions:
[tex]a^2+3b-2a^2=3a^2+3b-4a^2 \iff -a^2+3b=-a^2+3b[/tex]
[tex]a^2+3b-2a^2=3a^2+4b-4a^2 \iff -a^2+3b = -a^2+4b[/tex]
[tex]-a^2+3b-2a^2=3a^2+3b \iff -3a^2+3b = 3a^2+3b[/tex]
[tex]-a^3+3b+2a^2=3a^2+4b-2a^2 \iff -a^3+3b+2a^2=a^2+4b[/tex]
So, the only expression whose left and right hand sides are equal is the first.
