There are numerous information's already given in the question. Based on those information's the answer can be easily deduced.
Since temperature is inversely proportional to volume, so
T = k/V
30 = k/90
k = 2700
When the volume of the gas is 18 cm^3, then
T = 2700/18
= 150 degree centigrade.
I hope that this is the answer that you were looking for and the answer has come to your desired help.