X ^ -0.25=0.0003

 x ^ (-0.25) = 0.0003   →   1 / x^(1/4)  =  3 / 10000  →   x^(1/4)  = 10000 / 3

Take both sides to the 4th power

x = [ 10000 / 3 ] ^4  =  10000000000000000 / 81