6. 已知函数f(x)f(x)f(x)为奇函数,当x>0x>0x>0时,f(x)=log2(x+1)+axf(x)=\log _{2}(x+1)+axf(x)=log2(x+1)+ax,且f(−3)=af(-3)=af(−3)=a,则a=(a=(a=( )))