#Y=-sinx-cosx=-(sinx+cosx).#
#:. Y=-sqrt2(1/sqrt2sinx+1/sqrt2cosx)#
#=-sqrt2{sinxcos(pi/4)+cosxsin(pi/4)}#
#:. Y=-sqrt2sin(x+pi/4)#.
If, #A# is reqd. to be #+ve,# then, we can rewrite #Y# as,
#Y=sqrt2{(-1/sqrt2)sinx+(-1/sqrt2)cosx},# so that,
#Y=sqrt2{sinxcos(3pi/4)+cosxsin(3pi/4)}, or,#
#Y=sqrt2sin(x+3pi/4).#
Enjoy Maths.!