Determine the missing side length: [asy] size(4cm); void mra(pair u, pair v, pair w, real r){ draw(v (u-v)/length(u-v)*r--v (u-v)/length(u-v)*r (w-v)/length(w-v)*r--v (w-v)/length(w-v)*r); } pair a=(0,0); pair b=(28,0); pair c=(0,-21); draw(a--b--c--a); mra(c,a,b,2.5); label("28",(a b)/2,N); label("21",(a c)/2,W); [/asy]