`y=21/5 -2sin^2x+3cos2x=21/5-(1-cos2x)+3cos2x=16/5+4cos2x`
`-1 <= cos2x<=1`
`<=> -4<=4cos2x<=4`
`<=>-4/5 <= 16/5+4cos2x<=36/5`
`=> y_(min)=-4/5 ; y_(max)=36/5`
.
`y=2-4\sqrt(3+4sin3xcos3x)=2-4\sqrt(3+2sin6x)`
`-1 <= sin6x <=1`
`<=>1<=3+2sin6x <=5`
`<=>1<=\sqrt(3+2sin6x) <=\sqrt5`
`<=>-2>= 2-4\sqrt(3+2sin6x) >= -4\sqrt5+2`
`=> y_(min)=-4\sqrt5+2 ; y_(max)=-2`