Bad Design...

The 15 Ohm resistors won't have much effect and because of variations in the LEDs there's no guarantee the the current through all of the LEDs will be exactly the same, so the brightness may vary and there's even a slight chance that you'll blow one or more LEDs...

But here goes... 15mA through 15 Ohms is 0.225V. Add that to the LED voltage and you have 3.825V across each LED & resistor.

That leaves 1.175V across the unknown resistor.

You'll have 108 x 0.015A = 1.62 Amps through that resistor so we calculate 1.175V / 1.62A = 0.725 Ohms. (You'll probably have to use a 1-Ohm resistor.)

1.175V x 1.62A is 1.90W so you'd need a 2 Watt resistor. (The current, power, and brightness), will be a little lower with a 1-Ohm resistor.)