how to evaluate a quoted expression?

Hi,

after some derivations maxima gives me the following expression

%o15 ['diff(f(s2),s2,1)=(6*('diff(c(s2),s2,1))*t2)/(2*f(s2)*H-t2*H)]

then I define
f(s) := 1-s$
c(s) := beta*s$

and would like to get maxima reevaluate the original expression, so I run
''%o15   <- 2 single quotes, but I've tried ev() as well
but the derivatives (that are single quoted in %o15) are not evaluated.

How can I evaluate them as well?

thanks for your help!
Viktor