y=6cos^2a - √3sin2a
=6(cos2a+1)/2 - √3sin2a
=3(cos2a+1) - √3sin2a
=3cos2a- √3sin2a+3
=2√3(3/(2√3)*cos2a- √3/(2√3)*sin2a]+3
=2√3(√3/2*cos2a- 1/2*sin2a]+3
=2√3(cos2a*cosπ/6- sin2a*sinπ/6]+3
=2√3*cos(2a+π/6)+3