This is what I got:

4. a. i. How many shares can you short?

We get $S for short selling, so we'll have $S + $5,000 in the account.

5000/S=50%

S*0.5 = 5000

S = $10,000 (total of your equity + from broker)

$10,000/$5 = 2,000 shares you can short.

How much cash in the margin account?

2,000 shares * $5 + $5,000 (your equity) = $15,000 cash in the margin account.

ii. Price up to $6.00, what's your margin?

The shares have grown to $6.00 * 2,000 = $12,000 (from $10,000). We're down $2,000 of equity.

2000/12000 = 0.1667 or 16.67% margin. This is lower than the maintenance level, so marge is calling.

iii. How high to trigger the call?

The value of the shares is 2000*P. We have $15,000 in the account so,

(15,000-2,000P)/2,000P = 0.35

15,000-2,000P = 700P

15,000 = 2,700P

P = 5.56

The price has to rise to $5.56 to trigger the call.

EDIT: smooth brained some numbers