#cos^2 3x+3sin3x=3#
#=>1-sin^2 3x+3sin3x=3#
#=>sin^2 3x-3sin3x+2=0#
#=>sin^2 3x-2sin3x-sin3x+2=0#
#=>sin3x(sin3x-2)-(sin3x-2)=0#
#=>(sin3x-2)(sin3x-1)=0#
#sin3x=2->"not possible"#
So #sin3x=1=sin(pi/2)#
#=>3x=npi+(-1)^npi/2" where "ninZZ#
#=>x=(npi)/3+(-1)^npi/6" where "ninZZ#