Let phi: R-->R' be a ring homomorphism, I an ideal of R, J an ideal of R' and suppose that phi(I) is a subset of J. Let f and g be the natural homomorphisms, f:R-->R/I and g:R'-->R'/J defined by, f(r) = r+I for all r in R and g(r')=r'+J for all r' in R'.

Now, define a homomorphism h:R/I-->R'/J, prove that h is well-defined, that it is in fact a homomorphism and finally that for all x in R, hf(x)=gphi(x)