Okay so first you have to simplify the problem so that would be
15^(x-4)+8=100
Okay so know that it's simplified now you need to do the distributive property.
So it would be
15x-60+8=100
Now subtract 8 from 100
15x-60=92
Now you add 60 to 92
15x=152
Now dive 152 by 15 and that is your answer
X=10.133