9 tickets would cost $45 on Friday.
Since 3 tickets cost $21, hence the cost of each ticket is:
Cost of each ticket = $21 / 3 tickets = $7 per ticket
On Fridays, all tickets are discounted by $2, hence:
Cost for each ticket on Fridays = $7 - $2 = $5
The cost of 9 tickets on Friday = $5 per ticket * 9 tickets = $45
Hence 9 tickets would cost $45 on Friday.
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