The voltage accross each lamp is the same and 6V. So the current through
eash lamp is given by P = IV, here 18 = I.6, so I = 3 Amps.
Ohms law for a single lamp tells us that V=IR, so R=18/3 = 6 Ohms.
The total current supplied by the battery is 4 times the current through the
individual bulbs, so is 12 Amps. Now apply Ohm's law to the entire circuit to
get: 18 = 12.R, so R=1.5 Ohms is the resistence of the combined four lamps.