Calculating rotated relative positions on 2D plane

Question

For a game project, I need to calculate positions of items on a 2D plane relative to the camera.

Camera can be rotated, and it's coordinates refer to it's center.

In the attached images, a and b are items to display and c is camera. First image is absolute positions of items in the world. Second image is the result I'm trying to get.

Please note that rotation doesn't have to be 90 degrees, it can be any valid angle. I just used 90 degrees to keep this example simple.

Normally this is an easy task, but because camera can be rotated, translating to/from the center is not that easy, and it makes this problem complicated.

Here are inputs, and some javascript boilerplate code to test solutions:

var items = {
    a: {x: 3, y: 3},
    b: {x: 2, y: 4}
};

var camera = {
    x: 4, y: 4,
    width: 4, height: 4,
    rotation: 90
}

boilerplate code on jsfiddle.net

(Solutions don't have to be in javascript. Mathematical explanation is sufficient.)

Any help would be appreciated.