If +15 is also a a part of the power of 216 there will be no solution:
216^(-1.5t)=1-15
216^(-1.5t)=-14
-1.5t= 216 base log -14 as log with negative value(-14 in this problem) has no solution so,
I guess it is 216^(-1.5t+15)=1 right?
216^-1.5t+15=1
-1.5t+15=216 base log 1
-1.5t+15=0(Any log with value 1 of anybase has no value which is peopwely 0)
-1.5t=-15
t=15/1.5
t=10