Lets start by looking at a simpler example using numbers:
2^{3}2^{4 }
2^{3} = 8
2^{4} = 16
8 * 16 = 128
We could also write it out instead of using exponents:
2 * 2 * 2 * 2 * 2 * 2 * 2 = 2^{7 }=128
so we could say 2^{(3+4)}= 2^{(7)} = 128
The point is, we can just add the exponents since they have the same base (2)
(10x-1) + (10-3x) = 7x-9
so f(x) = 7x-9
2^{(7x-9)}