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