Find $a,$ such that the function $f(x)=4x\sqrt[\phantom{2}]{a-x^2}$ has the domain $-3,3.$

a$)a=-9$

b$)a=\sqrt[\phantom{2}]{3}$

c$)a=-\sqrt[\phantom{2}]{3}$

d$)a=9$

e$)a=3$