The mark-up is 50% of the cost, so the mark-up is 50% x 275.65277 (not 50/150 x 275.65277). (Mark-ups are always a % of cost, and this question specifically says so anyway 🙂 )
Part (b) is not calculating r to the third power (that would just be multiplying r by itself 3 times). In part (b) we know that r^3 = 0.343, therefore r = the third root of 0.343, which is 0.70 (I assume you have a scientific calculator – if not you need to buy one 🙂 )