Find the value of

$ \left(\frac{1}{7}\right)^{-4} + \left(\frac{1}{9}\right)^{-4} + \left(\frac{1}{5}\right)^{-4} $
$= 7^4 + 9^4 + 5^4 $
= 2401 + 6561 + 625
= 9587