#### Answer

$a=-5,3$

#### Work Step by Step

First, put all the terms on the left side. Then, to factor by trial and error, determine the two numbers that, when multiplied, equal the third term, and, when added, equal the coefficient of the middle term.
$a^2+2a=15$
$a^2+2a-15=0$
$(a+5)(a-3)=0$
$a+5=0$
$a=-5$
or
$a-3=0$
$a=3$. So
$a=-5,3$.