Answer
$x^{3a}+5x^{2a}-3x^a-15$
Work Step by Step
Using $(a+b)(c+d)=ac+ad+bc+bd$ or the FOIL Method and using the laws of exponents, the given expression, $
(x^a+5)(x^{2a}-3)
$, is equivalent to
\begin{array}{l}\require{cancel}
x^{a+2a}-3x^a+5x^{2a}-15
\\\\=
x^{3a}+5x^{2a}-3x^a-15
.\end{array}
