Beginner 'apply' question



I'm attempting to solve a simple equation in terms of known variables 
and then calculate the value of the unknown by applying the known.  For 
example, given a^2+b^2=c^2, a and c are the known variables with values 
of 3 and 5.  It seemed like the following would do the trick:

e1: a^2+b^2=c^2;
apply(rhs(first(solve (e1, b))), [3, 5]);

expecting to get 4 as an answer but instead get an error but don't yet 
know enough of maxima to troubleshoot it.

Thanks,
Phil