Answer
$a^2b^2-2ab-15$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $
(ab+3)(ab-5)
,$ simplifies to
\begin{array}{l}\require{cancel}
ab(ab)+ab(-5)+3(ab)+3(-5)
\\\\=
a^2b^2-5ab+3ab-15
\\\\=
a^2b^2-2ab-15
.\end{array}