Answer:
C'(2,-1)
Step-by-step explanation:
1) Transform point C(1,2) to polar coordinates.
C(1,2)=C(x,y)=C(r,tetha)=C(squareroot(5),63.44Degrees)
2) Substract 90 degress to get C'
C'=(squareroot(5),-26.56)
3) Transform point C' to cartesian coordinates.
C'=(x,y)=(squarerott(5)cos(-26.56),squareroot(5)sin(-26.56)
=C'(2,-1)