[tex]\left(a\left(a+b\right)-b\left(a+b\right)\right)
[/tex]
We take:
[tex]a\left(a+b\right)[/tex]
And aply the law of the exponents:
[tex]=a^2+ab[/tex]
we do the same process at the other side and we get:
[tex]=a^2+ab-ab-b^2[/tex]
Now:
[tex]+ab-ab=0[/tex]
So we get that:
[tex]=a^2-b^2[/tex]
