k^2+10k+16/k^2+13k+40 * k^2+5k/k^2-2k-8

I will assume you meant

(k^2+10k+16)/(k^2+13k+40) * (k^2+5k)/(k^2-2k-8)
= (k+2)(k+8)/[(k+8)(k+5)]*k(k+5)/[(k-4)(k+2)]
=k/(k-4) , where k ≠-8, -5, -2, 4