||'''Book'''|| Advanced Engineering Mathematics ||
 ||'''Author'''|| Erwin Kreyszig ||
 ||'''Edition'''|| 8th Edition ||

/* code_begins */

{{{
var('x,y')
y=function('y',x)
desolve(diff(y,x,2) + 9*y == cos(3*x), dvar = y) # i have assumed that the trig angle is cosine
}}}

/* code_ends */

 * '''Solution by''': 
   * Akash Singh, Student, CRCE