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.!