21．已知函数f(x)=a(ex−e−x)+bx+2f(x)=a(e^{x}-e^{-x})+bx+2f(x)=a(ex−e−x)+bx+2，(ab≠0)(ab\ne 0)(ab=0)，若f(h)=−2019f(h)=-2019f(h)=−2019，则f(−h)=f(-h)=f(−h)=[ 2023 ]．