  In this paper, we consider the entire solutions of nonlinear difference
equation $$f^3+q(z)\Delta f=p_1 e^{\alpha_1 z}+ p_2 e^{\alpha_2 z} $$ where $q$
is a polynomial, and $p_1, p_2, \alpha_1, \alpha_2$ are nonzero constants with
$\alpha_1\neq \alpha_2$. It is showed that if $f$ is a non-constant entire
solution of $\rho_2(f)<1$ to the above equation, then
$f(z)=e_1e^{\frac{\alpha_1 z}{3}}+e_2e^{\frac{\alpha_2 z}{3}}, $ where $e_1$
and $e_2$ are two constants. Meanwhile, we give an affirmative answer to the
conjecture posed by Zhang et al in [18].
