$($a$)$ When$$ the integral$$

I$=(3$x$+6)2$cos$(3$x$)$dxI$=(3$x$+6)2$cos$(3$x$)$dx

$$is calculated by using$$ integration by parts,$$ at a first step, one obtains$$

I$=(3$x$+6)2$I$=(3$x$+6)2-$ dxdx

$($b$)$ Using again an integration by parts one obtains$$

I$=$I$=+$C