  Suppose that $n$ is a positive integer. In this paper, we show that the
exponential Diophantine equation $$(n-1)^{x}+(n+2)^{y}=n^{z},\ n\geq 2,\
xyz\neq 0$$ has only the positive integer solutions $(n,x,y,z)=(3,2,1,2),
(3,1,2,3)$. The main tools on the proofs are Baker's theory and
Bilu-Hanrot-Voutier's result on primitive divisors of Lucas numbers.
