1) Current is defined as or if you have a Calculus based class .
2) I'm not sure of the problem here. Perhaps you are confusing the symbol for power, P, with the symbol for potential difference, V. (Many students get this mixed up at one point or another.) In any case you are looking for an equation for R in terms of I and V. Ohm's Law states that so
3) From the first problem we know that