(1) The first two parts should not cause any problems. To explain how the car fits in the garage, draw a spacetime diagram to illustrate what is going on.
(2) This question is easy, so no hint here!
(3) First write down the conservation of four- momentum equation for the collision. Now the trick here is to rearrange this expression in such a way that when you square it, the angle is eliminated. This will give you an expression for in terms of and . To obtain the final expression, you have to relate and to E and E'.
(4) The first part is straight out of the notes. Have a go at the rest of the question. A chocolate bar will go to the first person who presents me a perfect solution!!