I am going to explain this in words ( but just so you know this you can solve using a rotation matrix :) ).
We notice that rotating the figure 270 degrees about the origin is like rotating the figure 3 times 90. Now, rotating 90 degrees something lets say a point (x,y) will give you the point (-y,x), doing that process again we get a rotation of 180 degrees and (-y,x) will become (-x,-y), finally the last rotation will convert (-x,-y) to (y,-x).
Then, the original point (x,y) became (y,-x) after the 270 degrees rotation about the origin.
Then for any coordinates you can use this same result, for example for the coordinates of the triangle.