Let G be an angle. Show that the plane transformation Q=f(P) that maps the point P(x,y) to the point Q(x',y') where:

x' = x * cos(G) - y*sin(G)

y' = x* sin(G) + y*cos(G)

is in fact the rotation Ro,G.

I understand the meaning of G,Q,P, and the function. However, I don't know what to do with the x' and y'. I don't know how to start this proof. What is the strategy to do it? and how to do it?