3. Let $a$ be a non-zero real number, in the expansion of the binomial $(a x+1)^{5}$, the coefficient of the $x^{4}$ term is equal to the coefficient of the $x^{3}$ term, then the value of $a$ is $\qquad$ .
The coefficient of the $x^{4}$ term is $C_{5}^{1} a^{4}$, the coefficient of the $x^{3}$ term is $C_{5}^{2} a^{3}$, so $5 a^{4}=10 a^{3} \Rightarrow a=2$.