I also assume that "assume that the chocolate bar is flat" means that we disregard its thickness and treat it as two-dimensional. The idea is that if you fold over the four triangular areas then they just cover the inner rectangle. If that is the case, then the area cd of the outer rectangle (width c, height d) must be twice the area ab of the bar of chocolate. If the offset angle is , then and . Thus . If this is equal to 2ab then . From that, given a and b you can find , and hence c and d.