Chapter R - Elementary Algebra Review - R.4 Polynomials - R.4 Exercise Set - Page 961: 50

$3ab$

Removing the grouping symbols and then combining like terms, the given expression, $ (ab+x^2y^2)+(2ab-x^2y^2) ,$ simplifies to \begin{array}{l}\require{cancel} ab+x^2y^2+2ab-x^2y^2 \\\\= (ab+2ab)+(x^2y^2-x^2y^2) \\\\= 3ab+0x^2y^2 \\\\= 3ab .\end{array}