for a quadratic "expression"

i.e. ax^2 + bx + c

with any real and integral value of a, b and c

how to show that if delta (b square minus 4ac) is a perfect square

then such a quadratic expression can be factorized by cross method?

into something like (mx-n)(px-q) where m,n,p,q are integers.

I had been thinking about it for a whole night.

Hope someone can help