Based on given conditions,
[tex]a²b + 3 ab - (-5 a²b - 9 ab) [/tex]
[tex]→a²b + 3 ab + 5 a²b + 9 ab[/tex]
[tex]→(a²b + 5 a²b) + (3ab + 9 ab[/tex]
Now collecting the coefficients and adding them,
[tex](1 + 5)a²b + (3 + 9)ab[/tex]
[tex]→6 {a}^{2}b + 12ab[/tex]
Hence, the 6a²b + 12ab should be added.
