1. What is the difference between RELATIVE FREQUENCY and PROBABILITY.

Relative frequencies are usually obtained from experiments such as tossing a die 100 times or surveys, whereas probabilities are calculated from theory.

e.g Knowing that a normal coin will have a probability of 1/2 of coming up either heads or tails.

2. Can a probability be negative?

No. Probabilities always lie between (and including) 0 and 1.
An event with a probability of 1 is certain to happen. 
An event with a probability of 0 is impossible.

3. What sort of problems do tree diagrams help to solve?

Tree diagrams are useful for solving problems involving probabilities and problems requiring the number of combinations available for a sequence of events.