In Exercises $19$ through $24,$ find the average value of the given function $f(x)$ over the specified interval $axb.$

$19.f(x)=1-x^2$ over $-3x3$

$20.f(x)=x^2-3x+5$ over $-1x2$

$21.f(x)=e^{-x}(4-e^{2x})$ over $-1x1$

$22.f(x)=e^{2x}+e^{-x}$ over $0xln2$