you need to use it like a standard function - the maya help doesnt help at all in showing that

float $var = pow(2,4);
print $var

>> Gives
16

So your code would be:
Object.Attribute = 1.7 * ( pow(PathLength.input1X, -0.73));

EDIT: Looks like you asked about this elsewhere and got the right answer a few months ago:
https://www.highend2d.com/boards/lofi...p/t239400.html

Si