Question 746152
If we don't remember how phase shifts work we can always use the sum/difference identities:

{{{cos(x + 180)=cos(x)cos(180)-sin(x)sin(180)}}}

now

cos(180)=-1 and sin(180)=0


{{{cos(x + 180)=cos(x)*(-1)-sin(x)*(0)}}}


{{{cos(x + 180)=-cos(x)}}}


and in our case 


{{{cos(x)=sqrt(1-sin(x)^2)=sqrt(1-(3/5)^2)=4/5}}}


so 


{{{cos(x + 180)=-4/5}}}