some gmat math

i'll appreciate if anybody can help me on those:

1. The number of diagonals of a polygon of n Sides is given by the formula d=n(n-3). If a polygon has twice as many diagonals as sides, how many sides does it have?
a) 3
b) 5
c 6
d 7
e 8

2.If all of telephone extensions in a certain company must be even munbers and if each of them uses all four of the digits 1, 2, 3 and 6. what is the greatest number of 4-digit extensions that the company can have?
a) 4
6
12
16
e) 24

3. A certain faction is equivalent to 2/5 . If the numerator of the fracion is increased by 4 and the denominator is doubled, the new fraction is equivalent to 1/3 . what is the sum of the numberator and denominator of the original fraction?
a) 49
35
28
26
e) 21

which one ? and how to figure them out?
thanx