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HINT:
Use $$I=\int_a^bf(x)dx=\int_a^bf(a+b-x)dx$$
and $$2I=\int_a^bf(x)dx+\int_a^bf(a+b-x)dx=\int_a^b\left(f(x)+f(a+b-x)\right)dx$$
Observe that if
$\displaystyle g(x)=\sqrt{\ln(9-x)},$
$\displaystyle g(4+2-x)=\sqrt{\ln(9-(6-x))}=\sqrt{\ln(x+3)}$